Add theme variants

- Use CSS variables instead of inherence
- Rename default theme to "Light - Serif"
- Rename Black theme to "Dark - Serif"
- Rename "Sans-Serif" theme to "Light - Sans Serif"
- Add "System" theme that use system preferences: Dark or Light
- Add Serif and Sans-Serif variants for each color theme

http/request/context.go

@@ -64,7 +64,7 @@ func UserLanguage(r *http.Request) string {
 func UserTheme(r *http.Request) string {
 	theme := getContextStringValue(r, UserThemeContextKey)
 	if theme == "" {
-		theme = "default"
+		theme = "light_serif"
 	}
 	return theme
 }