Qt-material theme bug fix

[MikeSullivan7]

Issue

Closes #2209.

Description

When the Theme in the settings menu is changed to Fusion, the Stylesheet of the QApplication is reset correctly. The UI bugs were caused by the qt-material function apply_stylesheet changing the applications stylesheet and those changes persisting once the theme was changed back to Fusion.

Acceptance Criteria

Repeat the changes in the linked issue and check that the themes look as expected:

1. Start Mantid Imaging
2. Select File > Settings
3. Select an alternative theme to Fusion
4. Restart mantid Imaging
5. Select File > Settings
6. Select "Use OS Default"

Also change any and all combinations of settings in the Appearance Settings tab to check they are behaving correctly. MI should also correctly store and apply the last given settings upon startup.

Example screenshots are given here:

My default OS settings (I use dark mode!)

changing to a qt-material theme:

restarting MI and clicking the Use OS defaults checkbox:

Documentation

Release Note

[coveralls]

coverage: 72.721% (-0.01%) from 72.735%
when pulling 992c492 on 2209_qt_material_theme_bug_fixes
into 4d2ef3e on main.

[samtygier-stfc]

I could not trigger the original issue on linux.

But change looks good, and all functional on linux. So happy to approve.

[JackEAllen]

I can give this a secondary review on Monday if not merged in by then having found the original issue 🙂

[JackEAllen]

Looks good! Thanks for putting a fix in for this. I have tested on Linux and Windows.
I have replicated the bug on main on Windows and compared to the proposed changes which resolve the bug.