UPDATE: See also https://www.odoo.com/forum/help-1/can-i-have-a-different-theme-color-navbar-for-each-company-v15-ee-203360

You can create this Custom View, which overrides the WEB LAYOUT View:

Code to COPY / PASTE:

data inherit_id="web.layout"
    xpath expr="//body" position="inside"
        style
            .o_main_navbar {background-color: #5a7b87 !important;border-bottom: 0px;}
            .o_main_navbar .show .dropdown-toggle {background-color: #5a7b87;}
            .o_main_navbar > ul > li > a:hover, .o_main_navbar 
                   > ul > li > label:hover {background-color: #4a666f;}
            .o_main_navbar > a:hover, .o_main_navbar > a:focus, .o_main_navbar 
                   > button:hover, .o_main_navbar 
                   > button:focus {background-color: #4a666f;color: inherit;}
            .o_main_navbar > a:hover, .o_main_navbar > a:focus, .o_main_navbar 
                   > button:hover, .o_main_navbar 
                   > button:focus {background-color: #4a666f;color: inherit;}   
        style
    xpath
data

Note: you have to add the opening and closing angle brackets to the first 3 and last 3 lines, like in the screenshot.

This gives you a the following color scheme:

You can use the web to find a complimentary dark color / shade to the one you choose:

https://colorbase.app/colors/5a7b87