Hi,

You can add your css file in the path:static/src/css/style.css

you can create a file style.css in the above give path in your custom module and add the CSS style in the file

And add the path in asset bundle in your manifest'assets': {

        'web.assets_backend': [

            'module_name/static/src/css/style.css',

        ],}

Hope it helps

Hi, Your can add them to class have name .o_main_navbar. 
Reference document in https://www.odoo.com/documentation/17.0/developer/reference/frontend/assets.html.