A theme is the whole look of a website: its layout, its type, the shape of
every page.
Your website does not use one of these. It was built for your trade, by hand,
around the way you sell your work. Go to Admin, then Themes, and you
can browse what is on offer. None of it is a match for what you have.
To change your colors, use the Site Theme settings instead. Plugins
walks you through it.