Please be more explicit on what you want by 'defaults.' Do you mean use of HTML for mail composition? Or maybe view messages in HTML? Or something else? Thank you.

I think viewing the emails in HTML. When I open an email now they are just in a plaintext format. Not the original HTML of the email, which is making things tricky.

clicking view>message body as> original HTML should do it. If 'view' isn't visible, press the alt key.

I did try that first of all. The issue is I have been sent an email where the person highlighted in red but the email is just showing plain (white text on a black background)

Sorry, I now found the answer from looking in the right place from this thread prompting. Thanks! Settings > General > Fonts and colours.