Change css for --text-highlight-bg

[weizheheng]

• The current highlight is a little striking without opacity.
• Make use of opacity to make the highlighting from both outline and link to headings to look a little more subtle while still easy to read.

Signed-off-by: Marcus Heng marcushwz@gmail.com

[weizheheng]

Here are some screenshots,

Dark Mode (Before)

Dark Mode (After)

Light Mode (Before)

Light Mode (After)