Support selecting theme on the footer (#35741)

Fixes: https://github.com/go-gitea/gitea/pull/27576

routers/web/misc/webtheme.go

@@ -0,0 +1,42 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package misc
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/templates"
+	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/modules/web/middleware"
+	"code.gitea.io/gitea/services/context"
+	user_service "code.gitea.io/gitea/services/user"
+	"code.gitea.io/gitea/services/webtheme"
+)
+
+func WebThemeList(ctx *context.Context) {
+	curWebTheme := ctx.TemplateContext.CurrentWebTheme()
+	renderUtils := templates.NewRenderUtils(ctx)
+	allThemes := webtheme.GetAvailableThemes()
+
+	var results []map[string]any
+	for _, theme := range allThemes {
+		results = append(results, map[string]any{
+			"name":  renderUtils.RenderThemeItem(theme, 14),
+			"value": theme.InternalName,
+			"class": "item js-aria-clickable" + util.Iif(theme.InternalName == curWebTheme.InternalName, " selected", ""),
+		})
+	}
+	ctx.JSON(http.StatusOK, map[string]any{"results": results})
+}
+
+func WebThemeApply(ctx *context.Context) {
+	themeName := ctx.FormString("theme")
+	if ctx.Doer != nil {
+		opts := &user_service.UpdateOptions{Theme: optional.Some(themeName)}
+		_ = user_service.UpdateUser(ctx, ctx.Doer, opts)
+	} else {
+		middleware.SetSiteCookie(ctx.Resp, "gitea_theme", themeName, 0)
+	}
+}