https://www.odoo.com/forum/help-1/post/re-can-i-have-a-different-theme-color-navbar-for-each-company-v15-ee-203362/edit

Navbar - color specific to each Company

<!-- Note: the ID's of your Company records may not be 1 and 2, so adjust accordingly -->

<!-- Odoo 15 -->

<data inherit_id="web.layout">
  <xpath expr="//body" position="inside">
      <t t-if="request.httprequest.cookies.get('cids') and request.httprequest.cookies.get('cids')[0] == '1'">
        <style>.o_main_navbar {background-color: #57596F!important;border-bottom: 0px;}</style></t> 
      <t t-if="request.httprequest.cookies.get('cids') and request.httprequest.cookies.get('cids')[0] == '2'">
        <style>.o_main_navbar {background-color: #6B3C3C!important;border-bottom: 0px;}</style></t>
  </xpath>
</data>

<!-- Odoo 16 -->
<data inherit_id="web.layout">
  <xpath expr="//body" position="inside">
    <t t-if="request.httprequest.cookies.get('cids') and request.httprequest.cookies.get('cids')[0] == '1'">
      <style>.o_main_navbar {background: linear-gradient(45deg, #57596F, #57596F); 
                                         background-color: #57596F!important;border-bottom: 0px;}</style></t> 
    <t t-if="request.httprequest.cookies.get('cids') and request.httprequest.cookies.get('cids')[0] == '2'">
      <style>.o_main_navbar {background: linear-gradient(45deg, #6B3C3C, #6B3C3C); 
                                         background-color: #6B3C3C!important;border-bottom: 0px;}</style></t>
   </xpath>
</data>

Brilliant