Some --in-content CSS variables were renamed/removed

--in-content-link-color, --in-content-link-color-active and --in-content-link-color-hover where removed in favour of --link-color, etc. Bugzilla bug 1826829. This change has meant the links in "about:tor" are no longer purple.

--in-content-error-text-color was replaced with --text-color-error. Bugzilla bug 1869596. This change effects "about:torconnect" and "about:preferences#connection".