How to change colour of clicked links?

What theme are you using? Where did you download it from?

https://themeforest.net/item/directory-portal-wordpress-theme/3840053?WT.ac=solid_search_item&WT.seg_1=solid_search_item&WT.z_author=ait

Directory theme from Themeforest by AIT Themes

I’m sorry but as you are using a commercial theme, you need to seek support from the theme’s developer/vendor. We do not support commercial products here.
Try https://themeforest.net/user/ait

The problem is that the support from AIT is awful from my experience.

Could you suggest some generic CSS which would work in WordPress? Any suggestions would be incredibly helpful!

We do not support commercial themes as you paid for support when you bought the theme. It is not our place to keep theme vendors in business or deprive them of any income that they may derive from support services.

Well thanks.