Vertical Tabs: tab highlights when selecting multiple tabs are too subtle

[Brave-Matt]

Description

When vertical tabs are enabled, selecting multiple tabs (via Shift + Click) works but the tab highlights (showing which additional tabs are selected), while there, is not obvious. The tab highlights are too subtle and it can be hard to see what you've selected. It is particularly hard to tell when using the Light theme — it is a bit more apparent when dark theme is enabled.

No additional tabs selected:

All four tabs selected (light):

All four tabs selected (dark):

Brave version (brave://version info)

v1.60.88+

Version/Channel Information:

• Can you reproduce this issue with the current release? Y
• Can you reproduce this issue with the beta channel? Y
• Can you reproduce this issue with the nightly channel? Y

Miscellaneous Information:

Other users reporting the same issue: https://community.brave.com/t/visual-bug-with-multiple-tabs-selected-when-vertical-tabs-enabled/507521/4

It would be good to include this in color/theme update that is going out: https://github.com/brave/brave-browser/issues/31644

cc @rebron

[Coffeeling]

This is a regression as far as I can tell: For a while there was no effect at all on Beta branch at least. See #33012 (the beta has gotten these subtle multiselection effects from Nightly now)

I don't remember there being any issues with nonexistent or very subtle multiple tab selection effects at first, this is pretty new.