How to make a Template

Hello everyone !
I want to build some themes with html, css and javascript and I want to sell them online but how can i do to let the client custom the theme with his images, texts, colors that he want to put on the site. You know like a wordpress or a wix theme. I don’t find the answer anywhere cause I don’t know how to formulate correctly my sentence on google.

So if anyone has the answer let me know…

Thank you, have a good day. :smiling_face: