Wanting to add an image background to the navigation bar in 7.1.

Site URL: http://www.the37venue.com

I have been trying in vain to add an image background to the navigation of this site. I have uploaded the image in custom files and i'm pulling my hair out trying to figure this out.

Thank you in advance if you're able to help me! 

Replace your image url accordingly

.header#header {
    background-color: #353b3a;
    background: url(https://images.squarespace-cdn.com/content/v1/5f197bde37a20a5aae63e1ad/1605043495458-21WIV7SIKEBGYTCQCDKY/ke17ZwdGBToddI8pDm48kF9abE3_wCk-_Di9HguW_tF7gQa3H78H3Y0txjaiv_0fDoOvxcdMmMKkDsyUqMSsMWxHk725yiiHCCLfrh8O1z5QHyNOqBUUEtDDsRWrJLTmmt9rVnoh5SsI9ugICaYdmXKkg1E7F8jRYRDHeKot4IPA5d3Yw7TBNrpFTxxmsHDM/image-asset.jpeg) no-repeat;
    background-size: cover;
}