In my HTML5 skin, in Mobile Medium, I've set:
Main Page > Padding > Left - I've tried both 2 em and 20 px
Main Page > Padding > Left - Same
In "Preview" mode, this is displayed correctly - the required space is shown left and right.
However, if I built my online help and display it in a browser at the required width, text is flush with the left and right of the browser page.
P.