(1) Call "Markup Export" command (Ctrl+Shift+E)
(2) Pick your favorite Template and click "Edit" button.
(3) Find body { } session (or create one) and add in it the following line (you can, of course, vary that valour):
text-indent:10mm;
(4) Click "Save and Close" button.
(5) Click "Export" button and be happy! :)