We think you have already updated theme correctly.
The latest version is 1.9.0. But our developers didn't change the version number on styles.css file after update thene.
So you can see the v 1.8.0 continuously. Please go to wp-content/thenes/bigcart/styles.css and change the version number by 1.9.0.
If you have the other problems, please let us know again.
Best Regards, C-Themes