It seems that with a Forum or a community and without items in the sidebar, the forum and community width doesn't fit the page and the sidebar space remains white. How to make the community or forum occupy the full width?
In this case you have to put the width of the content to big value then it will fill the all available space and of course it will fit the browser screen on the smaller screens. But unfortunately it will probably enable the tablet.css file, because it's activating on the desktop screen smaller than inset+mainbody width.