Tabs inside groups unaffected by some changes

### Guidelines

- [x] I have read [how to report a new issue](https://github.com/Aris-t2/CustomCSSforFx/issues/4)
- [x] I have used the [latest release](https://github.com/Aris-t2/CustomCSSforFx/releases/latest)
- [x] I have set the `toolkit.legacyUserProfileCustomizations.stylesheets` in `about:config` to `true`

### Does the issue occur without any files inside /chrome/ folder?

No

### Description of the Issue

Tabs inside groups are unaffected by any direct "(parentclass) > .tabbrowser-tab" (for example: #tabbrowser-arrowscrollbox > .tabbrowser-tab) changes since the tabs get placed inside a "tab-group" element, this can be fixed by just adding an additional "(parentclass) > tab-group > .tabbrowser-tab" selector to the affected css file sections.

### Issue description, userChrome.css/userContent.css 'settings' the issue occurs with and steps to reproduce the issue

Any css files with "(parentclass) > .tabbrowser-tab" (not including "tab-group > .tabbrowser-tab", obviously), example: classic_squared_tabs "Lightweight theme on tabs" section.

### Does the issue occur with the most recent version of CustomCSSforFx (release/test/dev build)?

Yes, with the latest release

### Screenshots showing the issue


### System information

- OS & OS version: Not an system issue.
- DPI/HiDPI/scaling value, if not 100% (e.g. 125%, 150%...): Not an system issue.
- Firefox version: 138.0.3
- Firefox theme (e.g. default, build in light/dark, external theme + link): Not a theme issue.

Start with CSS sections containing `(parentclass) > .tabbrowser-tab`, including the `classic_squared_tabs` “Lightweight theme on tabs” section. Reproduce with tabs inside groups in Firefox 138.0.3, identify each affected selector, and verify that grouped tabs receive the same intended changes.

Written by the indexing model from the issue text.