Multilingual site with RTL

I have installed all the necessary modules required for language handling - i18n, locale, etc. - and have added an RTL language (Hebrew) to the site. Then, I created a 'Basic' page and made sure to mark its language as Hebrew. When I view this page, I expect the theme to pick up the fact that this page needs to be displayed right-to-left. However, in fact, the page is displayed as LTR. Can you please tell me how to set up the theme to support mixed language directions on the site?