I've created a new cutsom theme based on Blank theme, i have three storeviews ( for en, fr, and ar ).
I would like to know how to add css style for the currently selected language (Locale) ?
If I get this correctly, you want to have different CSS for every language.
Fist of all in you "themes.json" set param "localeOverwrites" to "true".
Then you need to follow this simple rule - path to overwriting file should be same as default file path when you remove "/l18n/lang_code".
For example, one of these files: