Fix url rebase when changing the theming color

[juliusknorr]

fix for #7704

taken from #7688

[skjnldsv]

You can drop my commits, there not needed! :)

Done :)

[MorrisJobke]

Tested and works 👍

[codecov]

Codecov Report

Merging #7710 into master will increase coverage by 16.29%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #7710 +/- ##
=============================================
+ Coverage 34.88% 51.18% +16.29% - Complexity 23636 24948 +1312 
=============================================
Files 1512 1605 +93 Lines 90182 94925 +4743 Branches 1376 1376 =============================================
+ Hits 31461 48584 +17123 + Misses 58721 46341 -12380

Impacted Files	Coverage Δ	Complexity Δ
apps/theming/lib/Controller/ThemingController.php	75.38% <100%> (+71.15%)	43 <0> (ø)	⬇️
lib/private/Server.php	81.43% <0%> (-0.12%)	134% <0%> (ø)
apps/files_external/lib/Lib/MissingDependency.php	100% <0%> (ø)	4% <0%> (?)
apps/files_external/lib/Command/Config.php	0% <0%> (ø)	15% <0%> (?)
apps/files_external/lib/Lib/Backend/Backend.php	96.66% <0%> (ø)	11% <0%> (?)
...external/lib/Controller/UserStoragesController.php	48.14% <0%> (ø)	12% <0%> (?)
...ternal/lib/Controller/GlobalStoragesController.php	36.25% <0%> (ø)	8% <0%> (?)
apps/files_external/lib/Settings/Admin.php	74.07% <0%> (ø)	4% <0%> (?)
apps/files_external/lib/Lib/IdentifierTrait.php	63.63% <0%> (ø)	8% <0%> (?)
apps/files_external/lib/Service/BackendService.php	90.27% <0%> (ø)	34% <0%> (?)
... and 614 more