New defcustom: accented background for tab-bar, tab-line

EDIT: This is done. Move to this comment to check the screenshots: #229 (comment 639571668)

I am experimenting with a new customisation option, which will affect the tab-bar and tab-line. Will also check with centaur tabs. Here is a demo, with more to follow if I eventually make this happen:

Screenshot_from_2021-07-30_10-50-20

Screenshot_from_2021-07-30_10-50-17

Edited by Protesilaos Stavrou