fix (addon/backgrounds): addon panel doesn't hide

[divslinger]

Fixes #6041

Instead of turning the preview transparent, addon-backgrounds should set the background color to the theme's default.
I added the background import and set background.content as the replacement for all transparent returns

[ndelangen]

This would make the preview background not respect the user's theme.

[codecov]

Codecov Report

Merging #6042 into next will decrease coverage by 1.86%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #6042      +/-   ##
==========================================
- Coverage   37.65%   35.78%   -1.87%     
==========================================
  Files         642      648       +6     
  Lines        9388     9520     +132     
  Branches     1364     1380      +16     
==========================================
- Hits         3535     3407     -128     
- Misses       5293     5536     +243     
- Partials      560      577      +17

Impacted Files	Coverage Δ
.../backgrounds/src/containers/BackgroundSelector.tsx	0% <0%> (ø)	⬆️
addons/a11y/src/components/Report/index.tsx	0% <0%> (-100%)	⬇️
addons/a11y/src/constants.ts	0% <0%> (-100%)	⬇️
addons/a11y/src/components/A11YPanel.tsx	0% <0%> (-94.45%)	⬇️
addons/a11y/src/components/Tabs.tsx	0% <0%> (-90.48%)	⬇️
addons/a11y/src/components/Report/Info.tsx	0% <0%> (-85.72%)	⬇️
addons/a11y/src/components/Report/Item.tsx	0% <0%> (-85%)	⬇️
addons/a11y/src/components/Report/Tags.tsx	0% <0%> (-71.43%)	⬇️
addons/a11y/src/components/Report/Rules.tsx	0% <0%> (-66.67%)	⬇️
addons/a11y/src/components/Report/Elements.tsx	0% <0%> (-63.64%)	⬇️
... and 38 more

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[codecov]

Codecov Report

Merging #6042 into next will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##             next    #6042   +/-   ##
=======================================
  Coverage   37.65%   37.65%           
=======================================
  Files         642      642           
  Lines        9388     9388           
  Branches     1364     1364           
=======================================
  Hits         3535     3535           
  Misses       5293     5293           
  Partials      560      560

Impacted Files	Coverage Δ
.../backgrounds/src/containers/BackgroundSelector.tsx	0% <0%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[ndelangen]

I refactored this addon quite a bit in my PR for migrating the core to typescript.

I incorporated this fix into that PR: 5592373

I also made sure the user's theme is respected.

[ndelangen]

I just pushed a little fix to this PR, because I want @GlitchPerfect to have this PR merged.

I really appreciate the effort.

I think this should be good to merge now.

I'll deal with the merge conflict in my api branch.

[shilman]

LGTM