Changed the div.thread class's hover color to match the hover color of the
activeThread, since var(--thread-active-bg) is a different shade from
var(--thread-hover-bg).
The difference is shown in the `themes.css` file:
```
--thread-hover-bg: var(--shades-black-90);
--thread-active-bg: var(--shades-black-80);
```