Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

.sidebar-left .widget-title{ 
    color: white;
    background-color: blue;
}

If you want to change the text, Navigate to Appearance > Widgets > Expand WoodMart WooCommerce layered nav and change the text:
https://ibb.co/FhFNFCF

Best Regards.