Add .dropdown-menu-dark

[mdo]

Fixes #30105.

Preview: https://deploy-preview-30171--twbs-bootstrap.netlify.app/docs/5.0/components/dropdowns/#dark-dropdowns

[MartijnCuppens]

I'm a bit worried that once we're adding some dark components, people will keep asking for support for other elements (eg. modals, pagination (which I declined in the past, #29892), form elements, ect... )

Once we switch to CSS variables, it 'll be way easier to colorize components.

[mdo]

Yeah, I agree on that concern, but we can still say no to most.

I mostly wanted to do this because of how prominent our dark navbar component is. Completing the style there and connecting the two components (navbar and dropdown) makes a lot of sense to me. The rest, I'm not as sold on, though we might as well discuss our options further beyond this.

[MartijnCuppens]

Ok, fine by me. Could you also switch the dropdowns of the dark navbars then (make sure to tackle the examples too)?

[mdo]

Could you also switch the dropdowns of the dark navbars then (make sure to tackle the examples too)?

Know I +1ed this back in February, but coming back to this I don't know how much I want to actually replace the other dark navbars. I think since this is a modifier class, it can stay as an option documented under the dropdowns.

Any aversions to merging as-is without further examples or docs examples?

[MartijnCuppens]

Any aversions to merging as-is without further examples or docs examples?

Now that we have CSS custom properties, this might be a good use case to use them. I think in we 'll need to update our other components to CSS custom properties (where suitable), adding this like might cause double work.

[mdo]

Thinking I'll leave this as-is for now and then come back in minor releases to CSS-variable-ify everything. Thoughts on that approach @MartijnCuppens?

[MartijnCuppens]

Job well done, @mdo!

[ffoodd]

LGTM

[XhmikosR]

I like it, I just don't know how we are going to proceed with dark components for all components in the future, but we'll discuss it later :)