fix(appearance): 完善深色模式和阅读页面对比度

- 区分按钮填充与图标颜色，统一书架、书源、搜索和阅读设置的选中态
- 调整编辑器语法高亮、键盘工具栏及统计卡片的浅深色显示
- 适配 RSS 正文和登录网页背景，保留书源自定义样式和页面状态
- 补充外观切换、页面对比度和管理页面的回归测试

App/Features/RSS/RSSWebReaderView.swift

@@ -41,6 +41,9 @@ struct RSSWebReaderView: UIViewRepresentable {
             controller.addUserScript(.init(source: inject, injectionTime: .atDocumentEnd, forMainFrameOnly: false))
         }
         let webView = WKWebView(frame: .zero, configuration: configuration)
+        webView.isOpaque = false
+        webView.backgroundColor = UIColor(ReadingTheme.canvas)
+        webView.scrollView.backgroundColor = UIColor(ReadingTheme.canvas)
         webView.navigationDelegate = context.coordinator
         webView.uiDelegate = context.coordinator
         webView.allowsBackForwardNavigationGestures = true