If you remove the blocks, then most themes should expand the main content to fill the screen
thanks for the response - to be honest, I've only looked at one or two so I may have just have picked the exceptions. I'll continue to investigate...
You need to use Clean or More themes in Moodle 2.7 onwards, as Moodle 2.6 may suffer from some missing CSS for certain layouts.
You can actually make the Frontpage 'all content' by removing the code that adds the blocks if you want. Then by enabling Frontpage Site Topic you can fill that section up with content of your choice, as well as have Site News and Course info, but you would need to know a little about HTML and Bootstrap to do that.
See this post of mine from a few months ago:
This is how Mary Cooch customised the Demo site.