Hi,

Please try with this code:

\
.vc_grid-filter > .vc_grid-filter-item > span{ font-family: 'Muli', sans-serif; font-size: 16px; }

And please note:

I think you should check Envato Support Policy for all items on themeforest here

However, item support does not include: 

- Customization services

While you did requirements us a lot of customize services questions. In next time, we will don't help you anymore question about custom work more because we can't provide supports like this for more than 11.000+ customers using this theme

I think you can understand this.

Best Regards,

PenciDesign