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

Sources/LegadoCore/RSS/RSSReadingPolicy.swift

@@ -34,6 +34,12 @@ public struct RSSReadingPolicy: Sendable {
 
 public struct RSSHTMLProcessor: Sendable {
     public static let defaultStyle = "img{max-width:100% !important; width:auto; height:auto;}video{object-fit:fill; max-width:100% !important; width:auto; height:auto;}body{word-wrap:break-word; height:auto;max-width: 100%; width:auto;}"
+    // Keep source CSS later in the cascade so intentional source styling wins.
+    public static let appearanceStyle = """
+    :root{color-scheme:light dark;}
+    body{background-color:#fcfcfc;color:#171717;}
+    @media(prefers-color-scheme:dark){body{background-color:#111113;color:#f2f2f2;}a{color:#9ecbff;}}
+    """
     public init() {}
 
     public func contentHTML(_ content: String, style: String?) -> String {
@@ -53,7 +59,7 @@ public struct RSSHTMLProcessor: Sendable {
     }
 
     private func injectStyle(into content: String, style: String) -> String {
-        let tag = "<style>\(style)</style>"
+        let tag = "<style>\(Self.appearanceStyle)</style><style>\(style)</style>"
         if let range = content.range(of: "</style>") {
             return String(content[..<range.upperBound]) + tag + String(content[range.upperBound...])
         }