Hi @yps104,
what @EmadBeshai told you is at least one solution.
However I highly recommend trying to use the theme's provided by the platform itself.
Try that first and change all the colours you want.
In other case you don't have to download yours but to upload a custom css file which yoou create somewhere on your desktop and upload it. There you would use CSS Selectors to further adjust colours, margins, padding - styling in general.
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Hope that helps.
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