Add CSS variables for Mermaid sequence diagrams

[sisp]

I've added CSS variables for Mermaid sequence diagrams to ease customization. I've also improved the note style to align better with the rest of the theme. Except for the note style, all other CSS variables use the previous styles as defaults.

The rationale of the CSS variables is this:

• Each sequence diagram component has its own namespace --md-mermaid-sequence-<component>-*.
• *-fg-color maps to text color – seems consistent with existing variable names
• *-bg-color maps to shape background color (fill)
• *-border-color maps to shape border color (stroke)
• *-line-color maps to line color (typically stroke, fill for arrowhead)

I've added a commit for each component to make the changes per component easier to check.

WDYT, @squidfunk @nejch?

Resolves #5824.

[squidfunk]

Thanks for the PR! Skimming through the PR, my first reaction is: looks really good! I'll review this PR in the coming days, so we can quickly merge it. Then, we should do the same for all other diagram types ☺️ Would you like to work on this as well?

[sisp]

Sounds great! Sure, once you've approved this approach, I can apply it to the other diagram types as well.

[squidfunk]

Looks great! We'd be happy to accept PRs for the other diagram types as well ☺️

[squidfunk]

Note that for now, we'll not going to advertise it on the docs, because I'm not yet entirely sure about the naming conventions. We might change the naming a little in the end, but for that to know we need the other types. Always better to decide when you see the whole picture. However, we'll ship it now with 9.2.7.