Bug 1597322 - Correctly set a:visited color in dark-mode. r=ewright

The light theme sets both visited and non-visited links to the same color
(`--blue-60`) by default, while the dark theme sets non-visited links to
`--blue-40` but did not specify a color for visited links, displaying them as
`--blue-60`. This resulted in inconsistent styling of pages written in XHTML
versus HTML, e.g. `about:protections` versus `about:newtab`. epang confirmed
that the desired behavior is to display links as `--blue-40`, visited or not.

Differential Revision: https://phabricator.services.mozilla.com/D60748

--HG--
extra : moz-landing-system : lando

toolkit/themes/shared/in-content/common.inc.css

@@ -140,6 +140,7 @@
     --in-content-link-color: var(--blue-40);
     --in-content-link-color-hover: var(--blue-50);
     --in-content-link-color-active: var(--blue-60);
+    --in-content-link-color-visited: var(--blue-40);
 
     --in-content-tab-color: var(--in-content-page-color);