WordPress already takes care of that for you. The third argument in your add_menu_page function, which reads edit_themes is the capability required to access the page. If the user doesn’t have that capability he can’t access the page, so WordPress won’t show him that option on the menu. Clever, no?
Now, from the code you mention on your comments, you’re restricting the page to user who can edit_posts. If that’s what you want, just use that on the add_menu_page function instead. Otherwise, it doesn’t make sense.
PS: If you’re planning to add a theme options page, I’d suggest using the add_theme_page function instead, which will nest it within the Appearance section of the menu.