Style readthedocs link previews in dark mode

[Liam-DeVoe]

Context: HypothesisWorks/hypothesis#4588

Readthedocs has a link preview plugin, which appears as white in furo's dark mode https://hypothesis.readthedocs.io/en/latest/:

Adding the following css styles fixes it:

body[data-theme="dark"] .tooltip .tooltip-content {
    background-color: var(--color-background-primary);
}

body[data-theme="dark"] .tooltip .arrow {
    background: var(--color-background-primary);
}

It would be great if furo shipped with something like this by default - though maybe it needs to wait for readthedocs/addons#490 for the css variables to stabilize?