after selecting the portrait theme from the reporting folder, remove the path information so only USLettePortrait.rdlx-theme is left. It seems that it takes the theme automatically from the right location.
I already tried this but the size of the page doesn't change.
If i manually change the size and save it, when i reopen the report the values are set back the original values.
Also when i try to change the PaperOrientation to Portrait, by the next reopening of the report the value is set back to landscape.