gui: use gtk_rc_parse to load our custom gtkrc

setting an environmental variable with our gtkrc causes applications
we launch (via gtk_show_uri) like firefox to inherit our custom
theme (provided a new firefox instance is being launched, instead
of just opening a new tab on an already existing instance).
(cherry picked from commit 18ba9b9f7f8a3236b90547f08901e2422b3255a8)