custom-push-theme is a compiled Lisp function in `custom.el'.

(custom-push-theme PROP SYMBOL THEME MODE &optional VALUE)

Record VALUE for face or variable SYMBOL in custom theme THEME.
PROP is `theme-face' for a face, `theme-value' for a variable.

MODE can be either the symbol `set' or the symbol `reset'. If it is the
symbol `set', then VALUE is the value to use. If it is the symbol
`reset', then SYMBOL will be removed from THEME (VALUE is ignored).

See `custom-known-themes' for a list of known themes.