[Feature] Native vertical tabs

[reesericci]

Is your feature request related to a problem? Please describe.

Firefox has rolled out vertical tabs natively in nightly and I'd like it if this theme would support them. https://connect.mozilla.org/t5/discussions/firefox-sidebar-and-vertical-tabs-try-them-out-in-nightly/td-p/63231

Describe the solution you'd like

Styling similar to the tab center reborn integration

Describe alternatives you've considered

Additional context

[Tuxino88]

bad support

[rafaelmardojai]

bad support

There's an initial theming for the sidebar in the nightly branch, but vertical tabs are not supported yet.