there are two ways,
first: if you want to remove default style, just dequeue them using wp_dequeue_style( 'style_file_name_here' ).
second: you can rename the existing file and create a new one with the same name, just place style hook info into new style file, like:
/*
Theme Name: Theme name
Author: Author name
*/
remember, the second way is unprofessional, but you can use this one as well.