Add theming system with light/dark mode support and CSS variables

[TheDormouse]

Resolves #441

Site will default to system setting, allowing for override which is stored in local storage.

Replaces many hardcoded values with CSS variables for easier theming.

[danielbrendel]

Wow, another nice PR! I will need some time for reviewing both of your PRs, as they are kinda big and I really want to take my time, so please be patient. But it's really nice to see someone getting into the core app!

[TheDormouse]

Wow, another nice PR! I will need some time for reviewing both of your PRs, as they are kinda big and I really want to take my time, so please be patient. But it's really nice to see someone getting into the core app!

No worries at all, please feel free to reach out with questions if needed. It's been fun to work with.

~ Marchy

[danielbrendel]

Hey, I've been starting to review your PR and I've got a few questions: - In your change to app/controller/_base.php you're going to load a default theme if there hasn't been one selected by the user. However it's by design that the system falls back to what is set via the admin dashboard -> media section. That particular logic is handled in app/views/layout.php:17-21. With the upcoming v5.3 users will have the opportunity to unselect any theme (even default one), so they are being served with what's adjusted in that particular section of the admin area. - Your implementation of ThemeModule::getCssVariablesInline() uses HTML strings, which I'm always a bit wary of. Can you elaborate how this is beneficial? Would it be possible to outsource this in a view script? - I'd put the navbar-item for bright/dark mode toggle between the logout and the location dropdown elements. Preferably with a bit of a gap at both sides, so it's clear that it's not part of the other regular menu actions. - Could you provide a small preview video or images that shows the workflow of toggling light/dark mode? For documentation and a visual presentation, this would help me as a reviewer. That's it for now. :D As this is a huge PR, we'll take our time to work through this. ;-) Really appreciate your coding style. Just one suggestion: For future contributions, it's better to avoid putting everything in a single commit (as stated in the contribution guidelines). While I do this myself sometimes, it makes things difficult if more people are involved. It will fasten the review if a PR is thematically splitted into multiple commits. For instance, server-side implementations, then frontend, then theme settings files, etc. If you have any questions, let me know!