Once in the website builder there is a option to 'change theme' on the right hand side of the page. Click into an 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 'republish' 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/