Also using the blue color for custom-variable-tag-face makes option names
similar to links. Maybe we should use the same colors that font-lock
defines for font-lock-constant-face and font-lock-variable-name and just
make them bold?
I think that would be worth to try.
Could you propose a better color?
I think your proposal to reuse the font-lock colors is
good. `font-lock-variable-name' seems appropriate to inherit in
custom-variable-tag-face' - at least as a mnemonic (and that is probably
good for a quick overview).
Not sure about what `custom-face-tag' could inherit from, but
font-lock-constant-face is not bad IMO.
One thing that it would be more logical, but another thing
how it would really look in Customize. Does this look good?