Add support for styles

[lenemter]

Closes #302
Closes https://github.com/orgs/elementary/discussions/589

Currently supports 3 styles:

Modern

Traditional

Transparent

[danirabbit]

I personally don’t want to add themes here. I think it’s better to just let themers add their own css overrides to their home folder. It’s kind of a can of worms imo

[alainm23]

I really like the idea of adding these three themes to the dock. Giving users official options for customization, makes the system more flexible and visually appealing without losing the elementary OS aesthetic. Plus, by keeping these themes in the official repository, we can ensure they work well and maintain quality, rather than relying on external themes that might not integrate as smoothly.

[lenemter]

@danirabbit I don't think shifting the customization to the community is a good thing. Officially supporting 3 most asked themes will cover all requests on dock theming.

[danirabbit]

The biggest issue is any time we want to make design changes now we have to support any themes and people always want more themes once they exist. Leonhard and I have already been talking about doing more with multitasking in the dock which will require more stylesheet changes and I personally don't want to have to come up with three different designs every time we want to add a new feature or make a change

I think it would be better to document how to provide your own CSS since the Discord community has already expressed interest in doing that and they seem to be happy with it being CSS and are having success there.

I do agree with merging changes that would make it easier to make themes like the branch that separates the background. I'm sure themers would be happy to be able to mess with 3D transforms etc there

But we also don't support themes anywhere else so I'm not sure why the Dock should be a special exception. Overall I'm pretty -1 to explicitly adding our own themes instead of letting the community do its own thing and providing documentation etc