If you are installing your theme from the WordPress theme installer at Appearance > Themes, but get a message that says “Are you sure you want to do this?”, most likely your web server is configured with PHP settings that are too low to allow the theme ZIP file to upload. Specifically, PHP is configured to a maximum file upload size limit at 8 megabytes or less. Our themes ZIP is anywhere between 4 – 12 megabytes in size. Thus, the web server is rejecting the upload. WordPress is giving you a rather ambiguous message in response. This is not a theme fault or bug.There two ways to solve this problem.
You can either get your web host to increase your PHP limits.
You can upload your theme to wp-content > themes folder via an FTP client.
Recommended PHP configuration limits are as follows:
max_execution_time 60
memory_limit 128M
post_max_size 32M
upload_max_filesize 32M
You can verify your PHP configuration limits by installing a simple plugin: WordPress phpinfo()