BUG:432027 Improve contrast and appearance of tabs

Small style update to the tabs UI. This should take care of this bug/improvement

[Request] Better distinction between selected and unselected document tabs https://bugs.kde.org/show_bug.cgi?id=432027

The actual CSS style was pretty heavily influenced from Pedro Reis's UI plugin https://github.com/veryprofessionaldodo/Krita-UI-Redesign/blob/master/krita-redesign/variables.py

Test Plan

This is just a CSS change, so I mostly just added a few elements to make sure it works. I also switched themes to make sure the themes updated.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.