Initial support for colorblindness-friendly themes (#30625)

Initial support for #25680

This PR only adds some simple styles from GitHub, it is big enough and
it focuses on adding the necessary framework-level supports. More styles
could be fine-tuned later.

services/webtheme/webtheme.go

@@ -0,0 +1,74 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package webtheme
+
+import (
+	"sort"
+	"strings"
+	"sync"
+
+	"code.gitea.io/gitea/modules/container"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/public"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+var (
+	availableThemes    []string
+	availableThemesSet container.Set[string]
+	themeOnce          sync.Once
+)
+
+func initThemes() {
+	availableThemes = nil
+	defer func() {
+		availableThemesSet = container.SetOf(availableThemes...)
+		if !availableThemesSet.Contains(setting.UI.DefaultTheme) {
+			setting.LogStartupProblem(1, log.ERROR, "Default theme %q is not available, please correct the '[ui].DEFAULT_THEME' setting in the config file", setting.UI.DefaultTheme)
+		}
+	}()
+	cssFiles, err := public.AssetFS().ListFiles("/assets/css")
+	if err != nil {
+		log.Error("Failed to list themes: %v", err)
+		availableThemes = []string{setting.UI.DefaultTheme}
+		return
+	}
+	var foundThemes []string
+	for _, name := range cssFiles {
+		name, ok := strings.CutPrefix(name, "theme-")
+		if !ok {
+			continue
+		}
+		name, ok = strings.CutSuffix(name, ".css")
+		if !ok {
+			continue
+		}
+		foundThemes = append(foundThemes, name)
+	}
+	if len(setting.UI.Themes) > 0 {
+		allowedThemes := container.SetOf(setting.UI.Themes...)
+		for _, theme := range foundThemes {
+			if allowedThemes.Contains(theme) {
+				availableThemes = append(availableThemes, theme)
+			}
+		}
+	} else {
+		availableThemes = foundThemes
+	}
+	sort.Strings(availableThemes)
+	if len(availableThemes) == 0 {
+		setting.LogStartupProblem(1, log.ERROR, "No theme candidate in asset files, but Gitea requires there should be at least one usable theme")
+		availableThemes = []string{setting.UI.DefaultTheme}
+	}
+}
+
+func GetAvailableThemes() []string {
+	themeOnce.Do(initThemes)
+	return availableThemes
+}
+
+func IsThemeAvailable(name string) bool {
+	themeOnce.Do(initThemes)
+	return availableThemesSet.Contains(name)
+}