fix #10054; mention .muted -> .text-muted in migration docs

BS3 commit 9b2ff3713ed31b2a9e297c4891443c385d180d0b
Run grunt dist on theme for some reason

BS3 commit 12a70cec888da0c41dba287e7207be10b5504680
add variables for heading font sizes; fixes 9921

BS3 commit 2b37d2b5333c0b10d5be53865a1a153d5ac4a4b5