This updates the issue summary table to display properly for both light

and dark modes. Light mode looks as it normally did (for the most part,

which I'll cover in a minute), using background colors to indicate the

status of issues. Dark mode changes the appearance a bit to use text

colors for identifying status rather than using background colors.
The table's styles come from a (currently in-progress) set of styles

from Ink for data tables. This mostly looks the same in light mode,

except that table headers now use var(--ink-p-header-bg), which gives

it a darker brown appearance in our case. This ends up being a nicer fit

for the review request box.
Tabs have had some changes as well. The bulky tab display always looked

a bit out of place here. We now adopt the style we use on

reviewboard.org's docs pages, where there's a thick border below the

tab, and no background or overall tab border. At least, that's the case

on light mode, where the colors nicely stand out. On dark mode, we do

introduce a subtle tab background as well.
In time, Ink will get a full data table component and a full tabs

component, and we'll probably switch to those then.

Tested on light and dark modes on both desktop and mobile views.