Vtiger provides an option to change the default theme of the menu bar appearing in the Portal. To do so, you just need to create a CSS file as per your business requirement and upload the CSS file to your server. After you upload, copy - paste the URL into the Customer Portal settings page.
Say, you have to change the color of the Menu bar from Black to Gold because your logo is not visible. Create a CSS file in the following format.
.menu .navbar.navbar-inverse { background: #FFD700 !important; }
Save the file and upload it to your server to generate a URL(eg, https://grasspods-doc.od2.vtiger.com/portal_css_sample.css). Copy-paste the URL in Vtiger Customer Portal, to see the changes in Portal.
You can choose the color code from the following website - https://www.w3schools.com/cssref/css_colors.asp