Hi Dora, the issue is because WordPress doesn’t handle sites changing from http to https very well at the moment.
It is fine if the site is https right from the start, but WordPress currently stores image links as absolute URLs, so for example an image uploaded when the site was http is stored in the WordPress database as http and if/when the site is changed to https the image URL is still only http.
The theme can only display content from WordPress, it is not allowed to make those changes, only plugins are allowed to do that.
So, we recommend installing the HTTP / HTTPS Remover plugin which converts all http to https and will fix the mixed content notice.