Iris here from Shopify Support. Just in case you were not aware and do speak (write) Dutch, this is our Dutch forum and any question can be written in Dutch. For questions in English, it is properly easier to consult our English Forum, as this is read and answered by a much larger group than here in the Dutch Forum.
The Shopify Design Forum also provides guidance on how to edit HTML, CSS, or Liquid to customize your online store. If you need help editing theme files, then try posting your question here. You can also search through the postings to see what other Shopify merchants are doing with their themes.
As the Booster theme is purchased outside of our ShopifyTheme Store, you might want to search for your answer on the help pages of Booster, here a link to their website. I also made a small video of how you can get in contact with their support:
Just to set expectations, third-party theme developers don't usually provide customizations, but should give support if there is a problem. If you need help customizing a paid theme, then you can hire a Shopify expert.
Hope you come right soon and good luck with your sales.