In the last update, all the base themes were updated with minor adjustments, probably preparing for the next Multiply update.
Besides that, now there is a CSS Validator built in in the Custom CSS page (where you add/edit your personal CSS). After the update, it was buggy but it seems that it's all fixed now. But there is a change now and it's permanent: now the Custom CSS will only be saved if
there is no errors in the code. What means that if your old code has errors and you try to edit it, maybe you won't be able to save the the changes until you fix all the errors in the code. Don't panic: the validator you show an error message and try to help you fix it.
Theme users: if you are using a custom theme and the theme you are trying to use won't save, try another one and/or contact the author in the same thread as the theme post asking her/him to fix the code.
Theme authors: double-check your code before publishing it to the group. This site may help you fix/clean your CSS code:
http://cleancss.com/