change text color per issue and conversation with ashoat.

It's hard to verify the diff when we don't know what was agreed. It is probably described on linear - the comment says that:

Talked IRL with @ashoat, settled on var(--shades-of-white-60); for the selected text color of the chat thread list item.

But it looks like on current master the value of --thread-color-read is --thread-color-read: var(--shades-black-60). Does the value of --thread-color-read changes in one of your diffs? This diff isn't a part of a stack so it shouldn't depend on anything else.