Modifying themes via uploading CSS

Software Support
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1

Hi,

I’m using the oldGregg theme in my Journal in OJS 3.1.1.2 and I’d like to upload a CSS file to change the background colours and fonts, which css properties do I need to change and are there any guides on how custom CSS sheets work?

Thanks.

Edit: To clarify on what I want to change, I want to replace the grey background with blue and change the font with a softer one.