The color box behind your forms is a theme setting, not part of the form itself. Open any page in the page editor, click Global theme styles in the left panel and go to Forms > Body > Background. Change the Background color to match your page, or set its opacity to 0 to make it transparent. The form title has its own box under Forms > Title > Background.
In Focus Plus you can also remove the box and its padding together: go to Forms > Padding and set Padding type to None. The free Focus theme has no padding option, so use the background color there.
Click Publish changes at the top right when you are done; theme settings apply to every form on your site.