How do I stop the post editor using theme styles?

Every time I change my theme the style of my post editor changes with it. By post editor I mean the TinyMCE visual editor that I use to write my articles not the editor that’s used for writing comments.

This would be very useful were it not for the styling of some themes interfering and clashing with the styling within the editor. For example:

• one theme caused everything within the editor to be centered – all text, all images… the published articles were unaffected by that centering but it made it difficult to edit old entries (I had to keep re-visiting published pages so I could see where items were and were not centered); and,
• two, another theme causes the editor to display with a very, very dark background colour that prevents me from seeing text. Surprisingly, the background colour of the editor is nothing like the background colour of my published pages.

How can I ensure that my TinyMCE editor always displays with a white background with default left-alignment regardless of the activated theme’s style?

Please, please help…