css: Fix night theme keyboard UI with popovers.

When interacting with popovers in the night theme using the keyboard
UI (e.g. the `i` menu for a message), the background color was
incorrectly white, resulting from the bootstrap `nav > li > a:focus`
rule.  We had already fixed this for `nav > li > a:hover`; we just
need to add `nav > li > a:focus` to the relevant block of CSS rules as
well.

Replaces #17195 and #17353.

Rewritten to use a cleaner solution by tabbott.

static/styles/night_mode.css

@@ -462,6 +462,7 @@ on a dark background, and don't change the dark labels dark either. */
     thead,
     .drafts-container .drafts-header,
     .recent_topics_container .recent_topics_header,
+    .nav > li > a:focus,
     .nav > li > a:hover,
     .subscriptions-container .subscriptions-header,
     .grey-box,