Optimize the "tabnav-tab" button

[zsdycs]

1. Optimize the style of highlighted "tabnav-tab" button when accessed by the "tab" key

There are two highlighting methods in the existing themes:

• outline
• box-shadow

The following example uses the two themes github-dark and github-dark-orange as examples.

• github-dark => outline
• github-dark-orange => box-shadow

github-dark before modification

“Write” button, when accessed by the "tab" key

“Preview” button, when accessed by the "tab" key

github-dark after modification

“Write” button, when accessed by the "tab" key

“Preview” button, when accessed by the "tab" key

github-dark-orange before modification

“Write” button, when accessed by the "tab" key

“Preview” button, when accessed by the "tab" key

github-dark-orange after modification

“Write” button, when accessed by the "tab" key

“Preview” button, when accessed by the "tab" key

2. Optimize the style when clicking the "tabnav-tab" button

This problem is because the "github-dark-orange" theme uses :focus for highlighting, it should use :focus-visible

This is the comparison between the "github-dark" theme and the "github-dark-orange" theme before modification:

"github-dark":

"github-dark-orange":

This is the "github-dark-orange" theme after modification:

[zsdycs]

fix: #519