If you have gone here => Dashboard => Presentation => Themes and clicked on a theme of your choice then you should be able to view your blog and find it there. Once changing themes you may have to rearrange your widgets. Also not that some themes do not have widgets. And as raincoasters has said if that isn't happening for you after you have cleared your browser: windows (with IE or FF) - ctrl F5 or macs (FF) - Cmd-shift-R and refreshed the page then send a feedback to staff using the button on the top right hand corner of any blog page.
I can't change colors in my theme, either. It just started a few minutes ago.
It is not possible to change the colours of your theme unless you have purchased a css customization upgrade.