How do you make a theme fill the whole screen? I do not want the big black boarder on each side.
Firstly, if you want to set up themes, I strongly recommend you make sure you are connected to your projector when you do so. When dealing with themes, OpenLP uses the size of the projection screen as a reference, and if your projector's resolution is not the same as your monitor's resolution, you are going to run into issues.
Secondly, OpenLP currently does not stretch background images, so make sure your background image is the same size as your projector's output (see why the previous point is important?)
Lastly, those thumbnails are not 100% accurate representations of what you'll see outputted to the projector because they are simple images generated at the time that the theme is edited. In most cases, when you get to projecting your lyrics, the theme will be fine.