When you have already “activated” a theme on your blog it will not longer appear in the “browsed” themes. It’s easy to change themes. And “tes” if you cange to another theme you can change vack to Tarski. Also note that there is a “Preview” function so you can see what the new theme will display like before you choose to click “Activate” or not.
The only advice I can give you is that prior to changing if you have special contents in text widgets that you have created you can drag them out of the sidebar box and drop them in the “Inactive Widgets” box first. In fact I find that it’s best to drag all the widgets I am using on a theme into the “Inactive Widgets” box prior to making a theme change. Then replacing them after I have activated a new theme is an quick and easy process.