You may use this code in Custom css https://prnt.sc/gqb1pd:
#mega-menu-wrap-main-menu {
width: 100%;
}
I would recommend you to update theme to the latest version http://prntscr.com/gkl0gf
https://www.8theme.com/demo/docs/woopress/#!/installation_upgrade
Before uploading theme files delete “woocommerce” folder, which is located in wp-content/themes/woopress/ to avoid problems with old files.
Don’t forget to create backup of your files and database before starting update process!
You need to enter your purchase code to activate our new theme version and skip Setup Wizard.
Also, update all required plugins https://www.8theme.com/download-plugins/ http://prntscr.com/gi54g8
Regards