Thank you it worked!!!
Topic summary
Missing CSS files (base.css, theme.css, theme.scss.liquid) in the Thalia-master theme were clarified as not used by this theme. Instead, the theme relies on assets/main.css.liquid, assets/style.css.liquid, and assets/custom.css.liquid. Adding custom CSS to main.css.liquid was recommended and deemed equivalent.
Button styling: The initially proposed selector (.hero-content__controls a) didn’t exist. A working rule was provided: body .wrap-btn-slide1 > a { background: #ffffff; color: #000000; }, which successfully turned the “Shop now” button white.
Header/search layout: CSS was provided to reposition elements near the cart for desktop via a media query targeting .wrap_header > div:first-child. Outcome not explicitly confirmed but acknowledged with thanks.
Removing grey lines: After a screenshot was shared, CSS was given to hide body .linedivide1 and .header-quick-links and remove borders on .wrap_header and .topbar for desktop.
Support scope and tools: Private help wasn’t available; a page builder app (GemPages) was suggested. The question about using GemPages while keeping Thalia-master wasn’t answered.
Status: Original CSS-file concern resolved; several UI tweaks addressed. Later question on which file was edited and theme–page builder compatibility remains unanswered. Images were relevant to diagnosing the grey lines.