Improve theme display (#30671)

Document: https://gitea.com/gitea/docs/pulls/180

![image](https://github.com/go-gitea/gitea/assets/2114189/68e38573-b911-45d9-b7aa-40d96d836ecb)

services/webtheme/webtheme_test.go

@@ -0,0 +1,37 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package webtheme
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseThemeMetaInfo(t *testing.T) {
+	m := parseThemeMetaInfoToMap(`gitea-theme-meta-info {
+	--k1: "v1";
+	--k2: "v\"2";
+	--k3: 'v3';
+	--k4: 'v\'4';
+	--k5: v5;
+}`)
+	assert.Equal(t, map[string]string{
+		"--k1": "v1",
+		"--k2": `v"2`,
+		"--k3": "v3",
+		"--k4": "v'4",
+		"--k5": "v5",
+	}, m)
+
+	// if an auto theme imports others, the meta info should be extracted from the last one
+	// the meta in imported themes should be ignored to avoid incorrect overriding
+	m = parseThemeMetaInfoToMap(`
+@media (prefers-color-scheme: dark) { gitea-theme-meta-info { --k1: foo; } }
+@media (prefers-color-scheme: light) { gitea-theme-meta-info { --k1: bar; } }
+gitea-theme-meta-info {
+	--k2: real;
+}`)
+	assert.Equal(t, map[string]string{"--k2": "real"}, m)
+}