Install user customizations of variable values specified in ARGS.
These settings are registered as theme `user'.
The arguments should each be a list of the form:
(SYMBOL EXP [NOW [REQUEST [COMMENT]]])
This stores EXP (without evaluating it) as the saved value for SYMBOL.
If NOW is present and non-nil, then also evaluate EXP and set
the default value for the SYMBOL to the value of EXP.
REQUEST is a list of features we must require in order to
handle SYMBOL properly. COMMENT is a comment string about SYMBOL.