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.

Was able to see description text for the web-based login pages.