Hi there,
Apologies I have been away on holiday for a while but am back now.
Editing the style.css file is enough to achieve this, I’m not sure why it is not working for you, maybe you are not being specific enough with your CSS element selections or try using the !important tag in your custom CSS.
As a side note, a new pro version is due to be released early February which has a custom CSS option.
All the best