There is 2 different causes for this issue.
If you are installing your theme from the WordPress theme dashboard, 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. The theme ZIP is currently about 34 mb. 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.
SOLUTION 1) You can either increase your PHP limits. Since every servers are configured differently, there is 3 ways to do this. You can try to do it yourself but if you don't have the skill, just ask your web host and they usually will do it for you.
To raise the PHP limits, try one of the 3 solutions below:
a) Add this in the .htaccess file of your server
php_value memory_limit 128M
php_value post_max_size 128M
php_value upload_max_filesize 128M
php_value max_execution_time 1800
b) Add this to wp-config.php
define(‘WP_MEMORY_LIMIT’, ’64M’);
c) Create a text file and name it php.ini then upload php.ini file in your root folder by FTP.
In the php.ini file, add:
upload_max_filesize = 64M
post_max_size = 64M
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If it still doesn't work, you can ask your web host to raise the upload and memory limit (Just tell them you are trying to upload a wordpress theme file that is around 35mb) OR jump to SOLUTION 2 below:
SOLUTION 2) You can upload your theme via an FTP client. Please read this article for more info: http://codex.wordpress.org/Using_Themes#Adding_New_Themes_Manually_.28FTP.29
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Recommended PHP configuration limits are as follows:
max_execution_time 600
memory_limit 256M
post_max_size 64M
upload_max_filesize 64M
You can verify your PHP configuration limits by installing a simple plugin called WordPress phpinfo().
ADDITIONAL INFO:
Sometimes, you might be trying to upload the whole theme package zip file (which contain the PSD, Licence, Theme, etc..), instead of only the theme zip file.
If that's the case, make sure you unzip the theme package zip file on your computer, then check if there is a /Theme/ folder. If so, make sure you are uploading the theme zip file that is located in that theme folder.