The default gallery shortcode in WordPress outputs a bunch of inline styling straight into the markup.
That often conflicts with one's attempts to style the gallery via an external CSS file.
Fortunately removing the inline block is trivial. Simply add the following line somewhere in your
add_filter( ‘use_default_gallery_style’, ‘__return_false’ );
Now you are free to style the gallery output without having first to reset any default.
Hopefully this will help someone.