Add dark themes to the backend and a theme switching support

[MassimilianoLattanzio]

Summary

Incorporates #5113

Compation SolidusAdmin PR #5511

This PR aims to add a dark theme switcher for the Solidus backend. The theme can be switched using a new select added in the left sidebar, and the choice is saved in local-storage.

System in dark mode	System in light mode

System in dark mode (new navbar)	System in light mode (new navbar)

This is just an easy way to invert theme colors as suggested by @elia following this StackOverflow example.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[MassimilianoLattanzio]

Some additional notes:

• There is an issue with the images using the dark theme that look darker than the relative ones in the default theme.
• The user preference can be saved in the DB instead of the cookie. Otherwise, whenever the user logs out will have to switch the theme again.
• Probably, the best way to add a new theme is to create a new bootstrap _variables_overrides file, but it will take a lot of time. We can use this way if the solution in this PR needs to be better.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9ed9a07) 91.57% compared to head (36c1acb) 88.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4999      +/-   ##
==========================================
- Coverage   91.57%   88.60%   -2.98%     
==========================================
  Files         188      630     +442     
  Lines        4415    15081   +10666     
==========================================
+ Hits         4043    13362    +9319     
- Misses        372     1719    +1347     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kennyadsl]

@MassimilianoLattanzio thanks for you contributions here, but I think we are not going to merge this right now. We are in the process of starting a complete redesign of the admin, and adding Dark Mode now would influence design processes and decisions that we don't want to anticipate now.

We will keep this on hold and use as a reference when it will be a good time to add this to the new admin. I hope this makes sense fo you as well, and in the meantime thanks again for your great work!

[MassimilianoLattanzio]

@MassimilianoLattanzio thanks for you contributions here, but I think we are not going to merge this right now. We are in the process of starting a complete redesign of the admin, and adding Dark Mode now would influence design processes and decisions that we don't want to anticipate now.

We will keep this on hold and use as a reference when it will be a good time to add this to the new admin. I hope this makes sense fo you as well, and in the meantime thanks again for your great work!

Sure! No problem at all! Makes sense to me.

Hope this can help other people and also for future updates of the admin UI.

I self assigned this review, so not considering my approval anymore

[waiting-for-dev]

🌃

[elia]

In order to avoid having to support changes after the release this is on hold until the new admin UI and dark-mode implementation will be defined (e.g. cookie vs session vs db).