Hello, @Oliver Theiß,
1. For the background color to change to White, Please copy and paste the below-given CSS code to XStore >> Theme Settings >> Theme Custom CSS >> Global Custom CSS, and check back your site after removing the browser cache.
.single-product .content-page {
background-color: #fff !important;
}
2. For removing one of the stars rows from the single product page, Please copy and paste the below-given CSS code to XStore >> Theme Settings >> Theme Custom CSS >> Global Custom CSS, and check back your site after removing the browser cache.
.single-product div#reviews form#commentform .comment-form-rating p.stars:nth-child(2) {
display: none !important;
}
3. For setting up the taxes, please read and follow the instructions written in the official WooCommerce documentation here: https://woocommerce.com/document/setting-up-taxes-in-woocommerce/#:~:text=Go%20to%3A%20WooCommerce%20%3E%20Settings%20%3E,Save%20changes.
Regards 8Themes Team.