@dmlphotography,

You can use a CSS code.

Insert this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea, save it, delete all caches, then reset your browser cache on forum front-end by pressing [CTRL] + [F5], if you're using Safari/Mac press [CMD] + [ALT] + [E].
If you're testing the result on mobile devices, please use a new private/incognito browser tab to avoid the hard cache, otherwise you'll not see any changes:

#wpforo p::-moz-selection { background: red !important; color:white !important;}
#wpforo p::selection { background: red !important; color:white !important;}

Hi @chris that worked to an extent but still getting a few sections which do not highlight well./ 

https://imgur.com/mvF7G8S

Seems to be when highlighting a link it turns dark - can I have some CSS to make that not happen ?!

@dmlphotography,

This all that we can help you on this issue. The things you are trying to achieve doesn't have direct relation to wpForo plugin, you will need some CSS expert to customize it.