HI @Apostolis_Broumas
Welcome to the community.
Great to hear you got your first sale, some with that kind of store wait for months.
But have to be honest and disagree with your " Everything is set pretty good, products, beautiful site, discounts everything"
First, about your products. For me, having rectangle area with a different background then material rarely works and looks good. In some cases. I prefer no background color, so graphics have no constraints. But that is just me, maybe.
Second, the prices, it looks a bit expensive.
“Everything is set pretty good.” well for me, it does not look like that, I got half English text, half Greek ( for menu, footer, add to cart, policies). All of your pages are very basic, one or two sections, not much content.
Homepage:
- You have two “Welcome” messages in the announcement bar. And that you see on all pages, one time is fine, but it should be better used; instead, use “Get any 2 items to get 25% off” and have link to all products.
- Menu you did not touch but you could have " T-Shirts" “Shorts”, “Tank Top” do provide quick navigation for customers to a specific collection.
- Your first section image/text is fine, but I would expect some info on your company, who are you, why to trust you? Would you buy from your store if yo uare customer? What is Team 210?
- Footer no links, no company info and do you really support all payments from icons in the footer? Klarna? No contact information, return policy
Collection page, no description, you do not need filters as not many products
Product page, just an image and some description. And saw “Short Sleeve T-shirt” do you really need 27 images? 
But no size guide, shipping, return info, care, and other sections. Customer reassurance, more trust signals, no reviews. And that contact use at iCloud email and “123 Main Street, Anytown, Country.” brings so much trust.
And contact information is important, first would you trust a store with just iCloud or Gmail email? You should at least use email with your domain. And no other info, not sure if in the privacy policy there is the actual address in Glyfada, as the page is in Greek. But no clear information on the site/footer. Company name, phone, address, and tax/vat or registration number.
The theme is fine, dark ones usually do not go as well as light ones, but it depends on the brand. I do not like that Add to cart button on prodcuct page, as outlined only. To me it is not that distinct as it should be.
And please do not think I am mean or want you to fail. I am just saying some things you are not aware. But they do influence your sales. You should spend some good time searching this community for store feedback posts and search “no sales” and " zero sales" to see the same topics from store owners and some tips that can help your own store.
Good luck and hope you do get more sales. But you do need more work.
