There is an option to 'Choose Your Theme' in the Wedding Website tile of your dashboard. You can also select/change your theme in the website builder where you need to click into the 'design' tab at the top of the editing panel. 

Click into an theme option to see a preview and choose a colour palette. To confirm your choice scroll to the bottom and click 'start using this theme'

If you have already made your website live you will need to chose 'publish' above the preview to ensure that your new theme shows on your live website.

You can see more information about how to create your own bespoke design theme here - - https://gettingmarried.co.uk/creating-a-bespoke-wedding-website/