This is May from Shopify.
You actually do not need to do any coding if you’d like to change your header color. You can accomplish this by editing your theme settings. Feel free to follow the steps below!
From your admin, go to ‘Online Store > Customize’.
On the editor bar to the left, click on the ‘Theme Settings’ tab, then, click on ‘Colors’.
Under the section ‘Header’, click on ‘Background’ and select your color from the color picker.
For future reference, if you’d like to learn more about what you can do with the Supply theme, you can check out our theme documentation. If you’d like to customize your theme, but you cannot find a tutorial in this document, then you’ll need to edit your theme code.
Let me know if this helps. Feel free to reply back to this thread or contact our 24/7 support, and we’ll happily help!