@Asafsky ,
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Go to Online Store → Theme → Edit code.
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Open your theme.liquid file
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Paste the below code before on theme.liquid
Result
If it helps you, please like and mark it as the solution.
Best Regards 
Topic summary
Goal: Make the homepage image banner on mobile fill the screen beneath the header.
Solution provided: Add Custom CSS to the banner section to set its height based on the viewport.
- Mobile rule: @media (max-width: 749px) { .banner { height: calc(100vh - 185px) !important; } }
- A general height rule for .banner was also shared alongside the mobile rule.
Implementation guidance: Another reply outlined how to apply CSS via Shopify’s editor:
- Online Store → Theme → Edit code → open theme.liquid
- Paste the CSS before .
- A screenshot was included showing the resulting larger banner.
Outcome: The original poster confirmed the solution works.
Notes for clarity:
- CSS “vh” means viewport height; using calc(100vh - [header height in px]) ensures the banner fills the remaining screen area below the header on mobile.
Status: Resolved; no further open questions.