Make the dark / light theme work with PWA

[McGiverGim]

Changed the way the dark-theme is loaded. Thanks to @VitroidFPV for the suggestion.

Now the PWA is dark theme only, with this it respects the user preference (light, dark or auto).

The less file shows a lot of changes, but is because it contains one indentation more and it has gone crazy. If you compare ignoring spaces the changes are only this, nothing more:

I've done some tests, but any test more will be appreciated. It must work both, under PWA and under Node.

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[nerdCopter]

• tested as working both PWA and NWjs. only quick-tested, did not check every tab.

[haslinghuis]

Found one issue using NWjs

PR:

Master:

[McGiverGim]

I don't know why is not equal... the problem is that the CSS of pid_tuning.css overwrites the background...
I fixed it anyway:

It seems the spacer between tabs is white and not black too... I will look into it... I suppose similar problem.

[McGiverGim]

@haslinghuis I think I've fixed it now :)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.8% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Do you want to test this code? Here you have an automated build:
Betaflight-Configurator-Linux
Betaflight-Configurator-Android
Betaflight-Configurator-Windows
Betaflight-Configurator-macOS
WARNING: It may be unstable and result in corrupted configurations or data loss. Use only for testing!