gtk:css-provider-load-named (provider name &optional variant)

provider -- a gtk:css-provider object
name -- a string for the theme name
variant -- an optional string for a variant to load, for example, a "dark" variant

Loads a theme from the usual theme paths. The actual process of finding the theme might change between releases, but it is guaranteed that this function uses the same mechanism to load the theme that GTK uses for loading its own theme.