In the Site Editor, click on your banner button to see its attributes. Click on the "Button Style" attribute. All Weebly site buttons have small and large size variants as well as a regular (default) and highlight button attribute (for each size) and you'll see all four button variants presented in the Button Style list.
Your theme's regular buttons have a transparent background and black button text and border. On hover, the button background changes to a translucent black (it's nearly transparent which is why it appears to be very light grey).
The theme's highlight buttons have white background and border with black button text. On hover, the background changes to transparent and the button text changes to white. The highlight button variant (large or small) is what you'll want to place in your banner image zone so just change your "Let the journey begin" button from the regular button to the highlight counterpart button (just click it in the Button Style list). Then move that button from the white section you created to your banner image zone and delete the white section.
