@yukon4
I don’t use any CSS related plugin so I cannot help in this case.
The trick is, that your custom CSS directive has to be after theme one regarding the same element. If not, it will be override by theme and you want exactly opposite, to override theme’s directive.
The simplest way I employ is to add custom style.css file to your theme.
Of course it is the best to create child theme before, to be on the safe side in case of theme upgrade.
You can do it without child theme but you have to recreate it every time theme is updated.
Another way is to use theme itself. Some themes have dedicated custom CSS option where you can do it.
I would not prefer to modify plugin CSS files because you’ll loose all modifications with next plugin update.