Untrack and ignore custom.el.

custom.el contains derived information that is often machine-specific
and is subject to non-semantic migration changes from differing emacs
versions, so treat it as non-authoritative -- prefer configuring
explicitly from theme file, sets.el and friends.
Theme: highlight-symbol-face
Theme: hl-paren-face
Theme: brighten font-lock-string-face.
Theme: show-paren-match.
Theme: Attempt to make magit diff colors sane. Round 13.
Theme: sort faces.
Advice (forward|backward)-paragraph to recenter page.