Authentication pages were hard-coding a color for paragraph text in the auth header. This wasn't noticible on most of the auth pages, because when people are logged out, we show the default light theme. For web-based RBTools authentication, we do have pages that the user could see in dark mode. This change fixes it to use the `--ink-p-fg-weak` color instead. Testing Done: Was able to see description text for the web-based login pages.