Use the following logo in the header of your form. The gray background is simply so you can see the logo - it is not part of the logo itself.
- Open the form editor.
- Click Theme.
- Scroll to the bottom of the available theme options and colors.
- Select + to customize the theme.
- Click the paint can icon.
- Enter #003865 as the hex value.
This will update the header background color as well as button colors.