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/context/context.go

@@ -230,6 +230,7 @@ func Contexter() func(next http.Handler) http.Handler {
 
 // HasError returns true if error occurs in form validation.
 // Attention: this function changes ctx.Data and ctx.Flash
+// If HasError is called, then before Redirect, the error message should be stored by ctx.Flash.Error(ctx.GetErrMsg()) again.
 func (ctx *Context) HasError() bool {
 	hasErr, ok := ctx.Data["HasError"]
 	if !ok {

services/forms/user_form.go

@@ -11,7 +11,6 @@ import (
 
 	auth_model "code.gitea.io/gitea/models/auth"
 	user_model "code.gitea.io/gitea/models/user"
-	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/services/context"
@@ -273,7 +272,7 @@ func (f *AddEmailForm) Validate(req *http.Request, errs binding.Errors) binding.
 
 // UpdateThemeForm form for updating a users' theme
 type UpdateThemeForm struct {
-	Theme string `binding:"Required;MaxSize(30)"`
+	Theme string `binding:"Required;MaxSize(255)"`
 }
 
 // Validate validates the field
@@ -282,20 +281,6 @@ func (f *UpdateThemeForm) Validate(req *http.Request, errs binding.Errors) bindi
 	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)
 }
 
-// IsThemeExists checks if the theme is a theme available in the config.
-func (f UpdateThemeForm) IsThemeExists() bool {
-	var exists bool
-
-	for _, v := range setting.UI.Themes {
-		if strings.EqualFold(v, f.Theme) {
-			exists = true
-			break
-		}
-	}
-
-	return exists
-}
-
 // ChangePasswordForm form for changing password
 type ChangePasswordForm struct {
 	OldPassword string `form:"old_password" binding:"MaxSize(255)"`

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)
+}