fix: fix hardcoded mouse browsing background 修正寫死的滑鼠瀏覽底色

[IepIweidieng]

Relevant issue: robertabcd/PttChrome#18

The support for customizing mouse browsing background was dropped in the following commit:
對自訂滑鼠瀏覽底色的支援已於下列 commit 中移除：

• 861847203bb2ad82f02a9b1e8aef113b46d71acf "Refactor out easy reading and rendering"

The functionality is reimplemented with the use of a new CSS variable --highlightBG.
該功能已以使用新 CSS 變數 --highlightBG 重新實作。

Screenshots