You can always 'manually' deactivate the current theme by FTP. Please go to your
/wp-content/themes folder, and rename the
Archive1 folder name to something like
Archive1-wrong. Actually it doesn't matter what you are renaming it to, it just matters that you rename it, this way WordPress cannot find the current theme, and fall back to other theme you have in the folder, usually to one of the twenty... themes.
At this point you should be able to access the admin section. To get rid of the error, you can always deactivate, delete and download again the theme, but this means that you have to start from the beginning also. If you need help finding the error in the code, please upload the functions.php file to Dropbox or Google drive and share the link here, I am happy to take a look for you :-)
Toth Balint BT