Scrollbar colour, Custom Theme import/export, ThemeAdjuster UI adjustments

[D1firehail]

What did you fix?

• Changed Scrollbar Colour
  • Dimon thought the new settings from Various features related to reward choice, some other fixes #263 were somewhat concealed due to the settings menu scrollbar blending in with the rest of the window. I think this colour change makes it sufficiently obvious without being too distracting
  • If anyone has a better idea (or colour), I'm open to suggestions
• ThemeAdjuster changes
  • Add import/export functionality
  • Add some tooltips
  • Add basic instructions below image
  • Images are immediately shown on load

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Reproduction steps

Scrollbar colour

1. Look at any window with a scrollbar

ThemeAdjuster export

1. Open Theme Adjuster
2. Adjust filters as desired, or leave them be
3. Click export button
4. Copy the contents of the textbox wherever you want

ThemeAdjuster import

1. Open Theme Adjuster
2. Paste an export into the textbox (or from other sources, as long as it has all required entries)
3. Click import button
4. See your custom filter get set to the imported one
   • Invalid or incomplete import attempt should throw an error and change none of the settings

ThemeAdjuster image load

1. Load image using either of the relevant buttons
2. See if image showed up
3. If image didn't show up, check if the image existed and if debug.log contains any errors

Other ThemeAdjuser Changes

1. Just look at ThemeAdjuser, maybe hover your mouse over a few elements

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Evidence/screenshot/link to line

Considerations

• Does this contain a new dependency? [No]
• Does this introduce opinionated data formatting or manual data entry? [No]
• Does this pr include updated data files in a separate commit that can be reverted for a clean code-only pr? [No]
• Have I run the linter? [No]
• Is is a bug fix, feature request, or enhancement? [Feature/Enhancement]

[dimon222]

LGTM