Twenty Fifteen: update scroll functionality to make sure child menu items are always reachable.

Merges [30838] to the 4.1 branch.

props iamtakashi.
see #30674.
Built from https://develop.svn.wordpress.org/branches/4.1@30938


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30927 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: fallback hex color for no-comments text should come first.

Merges [30910] to the 4.1 branch.

props iamtakashi.
see #30730.
Built from https://develop.svn.wordpress.org/branches/4.1@30939


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30928 1a063a9b-81f0-0310-95a4-ce76da25c4cd