feat: Add OpenCode-compatible theme loader

Add support for loading and applying OpenCode-compatible themes via URL.
Fetches theme JSON, resolves color references, generates CSS variables
and Tailwind class overrides to apply the theme seamlessly.

Features:
- Add theme_url config field under site section
- Fetch and parse OpenCode theme.json format
- Generate CSS custom properties (--theme-*) for all theme colors
- Generate Tailwind class overrides to apply theme colors
- Support both light and dark modes
- Template.CSS type for safe CSS injection

Example usage:
  site:
    theme_url: "https://raw.githubusercontent.com/anomalyco/opencode/.../opencode.json"

Theme schema: https://opencode.ai/theme.json

4 files changed, 312 insertions, 2 deletions

diff --git a/internal/config/config.go b/internal/config/config.go
index f3c35e3..68a88f7 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -40,6 +40,7 @@ type SiteConfig struct {
 	Description string `yaml:"description"`
 	Logo        string `yaml:"logo"`
 	Favicon     string `yaml:"favicon"`
+	ThemeURL    string `yaml:"theme_url"` // URL to OpenCode-compatible theme JSON
 }
 
 // ServerConfig contains HTTP server settings
diff --git a/internal/server/server.go b/internal/server/server.go
index 8681873..6d76fa0 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -17,6 +17,7 @@ import (
 	"github.com/Fuwn/kaze/internal/config"
 	"github.com/Fuwn/kaze/internal/monitor"
 	"github.com/Fuwn/kaze/internal/storage"
+	"github.com/Fuwn/kaze/internal/theme"
 )
 
 //go:embed templates/*.html
@@ -126,8 +127,9 @@ type PageData struct {
 	TickMode           string // ping, minute, hour, day
 	TickCount          int
 	ShowThemeToggle    bool
-	Timezone           string // Timezone for display
-	UseBrowserTimezone bool   // Use client-side timezone conversion
+	Timezone           string       // Timezone for display
+	UseBrowserTimezone bool         // Use client-side timezone conversion
+	ThemeCSS           template.CSS // OpenCode theme CSS (safe CSS)
 }
 
 // GroupData contains data for a monitor group
@@ -190,6 +192,19 @@ func (s *Server) handleIndex(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Load OpenCode theme if configured
+	var themeCSS template.CSS
+	if s.config.Site.ThemeURL != "" {
+		resolvedTheme, err := theme.LoadTheme(s.config.Site.ThemeURL)
+		if err != nil {
+			s.logger.Warn("failed to load theme", "url", s.config.Site.ThemeURL, "error", err)
+		} else if resolvedTheme != nil {
+			// Generate CSS variables and Tailwind mappings
+			cssString := resolvedTheme.GenerateCSS() + resolvedTheme.GenerateTailwindMappings()
+			themeCSS = template.CSS(cssString)
+		}
+	}
+
 	// Build page data
 	data := PageData{
 		Site:               s.config.Site,
@@ -198,6 +213,7 @@ func (s *Server) handleIndex(w http.ResponseWriter, r *http.Request) {
 		ShowThemeToggle:    s.config.Display.ShowThemeToggle != nil && *s.config.Display.ShowThemeToggle,
 		Timezone:           s.config.Display.Timezone,
 		UseBrowserTimezone: s.config.Display.Timezone == "Browser",
+		ThemeCSS:           themeCSS,
 	}
 
 	overallUp := true
diff --git a/internal/server/templates/index.html b/internal/server/templates/index.html
index db4c61a..6bdfeff 100644
--- a/internal/server/templates/index.html
+++ b/internal/server/templates/index.html
@@ -11,6 +11,12 @@
     <link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>🎐</text></svg>">
     {{end}}
     <link rel="stylesheet" href="/static/style.css">
+    {{if .ThemeCSS}}
+    <style>
+        /* OpenCode Theme */
+{{.ThemeCSS}}
+    </style>
+    {{end}}
     <script>
         // Theme detection
         if (localStorage.theme === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
diff --git a/internal/theme/theme.go b/internal/theme/theme.go
new file mode 100644
index 0000000..cddcaa4
--- /dev/null
+++ b/internal/theme/theme.go
@@ -0,0 +1,287 @@
+package theme
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// OpenCodeTheme represents an OpenCode-compatible theme JSON structure
+type OpenCodeTheme struct {
+	Schema string                       `json:"$schema"`
+	Defs   map[string]string            `json:"defs"`
+	Theme  map[string]OpenCodeThemeMode `json:"theme"`
+}
+
+// OpenCodeThemeMode represents dark/light mode values for a theme property
+type OpenCodeThemeMode struct {
+	Dark  string `json:"dark"`
+	Light string `json:"light"`
+}
+
+// ResolvedTheme contains the resolved color values for both modes
+type ResolvedTheme struct {
+	Dark  map[string]string
+	Light map[string]string
+}
+
+// LoadTheme fetches and parses an OpenCode theme from a URL
+func LoadTheme(themeURL string) (*ResolvedTheme, error) {
+	if themeURL == "" {
+		return nil, nil // No theme specified, use default
+	}
+
+	// Fetch the theme JSON
+	client := &http.Client{
+		Timeout: 10 * time.Second,
+	}
+
+	resp, err := client.Get(themeURL)
+	if err != nil {
+		return nil, fmt.Errorf("failed to fetch theme from %s: %w", themeURL, err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("failed to fetch theme: HTTP %d", resp.StatusCode)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read theme response: %w", err)
+	}
+
+	// Parse the theme JSON
+	var ocTheme OpenCodeTheme
+	if err := json.Unmarshal(body, &ocTheme); err != nil {
+		return nil, fmt.Errorf("failed to parse theme JSON: %w", err)
+	}
+
+	// Resolve color references
+	resolved := &ResolvedTheme{
+		Dark:  make(map[string]string),
+		Light: make(map[string]string),
+	}
+
+	for key, mode := range ocTheme.Theme {
+		// Resolve dark mode color
+		darkColor := resolveColor(mode.Dark, ocTheme.Defs)
+		resolved.Dark[key] = darkColor
+
+		// Resolve light mode color
+		lightColor := resolveColor(mode.Light, ocTheme.Defs)
+		resolved.Light[key] = lightColor
+	}
+
+	return resolved, nil
+}
+
+// resolveColor resolves a color value, handling references to defs
+func resolveColor(value string, defs map[string]string) string {
+	// If it starts with #, it's already a color
+	if strings.HasPrefix(value, "#") {
+		return value
+	}
+
+	// Otherwise, look it up in defs
+	if color, ok := defs[value]; ok {
+		return color
+	}
+
+	// Fallback to the value itself
+	return value
+}
+
+// GenerateCSS generates CSS custom properties from a resolved theme
+func (t *ResolvedTheme) GenerateCSS() string {
+	if t == nil {
+		return ""
+	}
+
+	var css strings.Builder
+
+	// Generate CSS variables for dark mode
+	css.WriteString(":root.dark {\n")
+	for key, color := range t.Dark {
+		cssVar := toCSSVariableName(key)
+		css.WriteString(fmt.Sprintf("  %s: %s;\n", cssVar, color))
+	}
+	css.WriteString("}\n\n")
+
+	// Generate CSS variables for light mode
+	css.WriteString(":root, :root.light {\n")
+	for key, color := range t.Light {
+		cssVar := toCSSVariableName(key)
+		css.WriteString(fmt.Sprintf("  %s: %s;\n", cssVar, color))
+	}
+	css.WriteString("}\n")
+
+	return css.String()
+}
+
+// toCSSVariableName converts a theme key to a CSS variable name
+// e.g., "background" -> "--theme-background"
+func toCSSVariableName(key string) string {
+	// Convert camelCase to kebab-case
+	var result strings.Builder
+	result.WriteString("--theme-")
+
+	for i, c := range key {
+		if i > 0 && c >= 'A' && c <= 'Z' {
+			result.WriteRune('-')
+			result.WriteRune(c + 32) // Convert to lowercase
+		} else {
+			result.WriteRune(c)
+		}
+	}
+
+	return result.String()
+}
+
+// GetColorMapping returns a mapping of semantic names to CSS variable names
+func GetColorMapping() map[string]string {
+	return map[string]string{
+		// Status colors
+		"status-up":       "var(--theme-success)",
+		"status-degraded": "var(--theme-warning)",
+		"status-down":     "var(--theme-error)",
+
+		// Text colors
+		"text-primary":   "var(--theme-text)",
+		"text-secondary": "var(--theme-text-muted)",
+
+		// Background colors
+		"bg-primary":   "var(--theme-background)",
+		"bg-secondary": "var(--theme-background-panel)",
+		"bg-tertiary":  "var(--theme-background-element)",
+
+		// Border colors
+		"border-primary": "var(--theme-border)",
+		"border-subtle":  "var(--theme-border-subtle)",
+		"border-active":  "var(--theme-border-active)",
+
+		// Accent colors
+		"accent-primary":   "var(--theme-primary)",
+		"accent-secondary": "var(--theme-secondary)",
+	}
+}
+
+// GenerateTailwindMappings generates CSS that maps theme variables to Tailwind classes
+func (t *ResolvedTheme) GenerateTailwindMappings() string {
+	if t == nil {
+		return ""
+	}
+
+	return `
+/* Override Tailwind classes with OpenCode theme colors */
+
+/* Background colors */
+.bg-white,
+.dark .bg-neutral-900 { 
+	background-color: var(--theme-background) !important; 
+}
+
+.bg-neutral-50,
+.bg-neutral-100,
+.dark .bg-neutral-950,
+.dark .bg-neutral-800 { 
+	background-color: var(--theme-background-panel) !important; 
+}
+
+.bg-neutral-200,
+.dark .bg-neutral-700 { 
+	background-color: var(--theme-background-element) !important; 
+}
+
+/* Text colors */
+.text-neutral-900,
+.text-neutral-950,
+.dark .text-white,
+.dark .text-neutral-50 { 
+	color: var(--theme-text) !important; 
+}
+
+.text-neutral-500,
+.text-neutral-600,
+.text-neutral-700,
+.dark .text-neutral-300,
+.dark .text-neutral-400,
+.dark .text-neutral-500 { 
+	color: var(--theme-text-muted) !important; 
+}
+
+/* Border colors */
+.border-neutral-200,
+.border-neutral-300,
+.dark .border-neutral-700,
+.dark .border-neutral-800 { 
+	border-color: var(--theme-border) !important; 
+}
+
+.divide-neutral-200,
+.dark .divide-neutral-800 { 
+	border-color: var(--theme-border) !important; 
+}
+
+/* Status colors - up/success */
+.bg-emerald-500.rounded-full,
+.bg-emerald-500.flex-shrink-0 { 
+	background-color: var(--theme-success) !important; 
+}
+
+.text-emerald-500,
+.text-emerald-600,
+.dark .text-emerald-400 { 
+	color: var(--theme-success) !important; 
+}
+
+/* Status colors - down/error */
+.bg-red-500.rounded-full,
+.bg-red-500.flex-shrink-0 { 
+	background-color: var(--theme-error) !important; 
+}
+
+.text-red-500,
+.text-red-600,
+.dark .text-red-400 { 
+	color: var(--theme-error) !important; 
+}
+
+/* Status colors - degraded/warning */
+.bg-yellow-500.rounded-full,
+.bg-yellow-500.flex-shrink-0 { 
+	background-color: var(--theme-warning) !important; 
+}
+
+.text-yellow-500,
+.text-yellow-600,
+.dark .text-yellow-400 { 
+	color: var(--theme-warning) !important; 
+}
+
+/* Neutral/unknown status */
+.bg-neutral-400.rounded-full { 
+	background-color: var(--theme-text-muted) !important; 
+}
+
+/* Hover states */
+.hover\:bg-neutral-100:hover,
+.dark .hover\:bg-neutral-900:hover { 
+	background-color: var(--theme-background-element) !important; 
+}
+
+.hover\:bg-neutral-200:hover,
+.dark .hover\:bg-neutral-800:hover { 
+	background-color: var(--theme-border) !important; 
+}
+
+/* Link colors */
+a.text-blue-600,
+a.dark\:text-blue-400 {
+	color: var(--theme-primary) !important;
+}
+`
+}