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 17 -->
<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>

<!-- Odoo 18.0 and Odoo 19.0 -->
<data inherit_id="web.layout">

<xpath expr="//body" position="inside">
    
    <t t-set="cookie_cids" t-value="request.httprequest.cookies.get('cids')"/>
    <t t-if="cookie_cids">
        <t t-set="active_cid" t-value="int(str(cookie_cids).replace('-', ',').split(',')[0])"/>
    </t>
    <t t-else="">
        <t t-set="active_cid" t-value="request.env.user.company_id.id"/>
    </t>
    <t t-set="target_company" t-value="request.env['res.company'].sudo().browse(active_cid)"/>
    <t t-set="color_map" t-value="{          0: '#8E44AD', 1: '#E74C3C', 2: '#F39C12', 3: '#F1C40F',           4: '#3498DB', 5: '#D35400', 6: '#C0392B', 7: '#1ABC9C',           8: '#34495E', 9: '#E91E63', 10: '#2ECC71', 11: '#9B59B6'      }"/>
    <t t-set="color_int" t-value="target_company.color or 0"/>
    <t t-set="bar_color" t-value="color_map.get(color_int, '#8E44AD')"/>

    <div t-if="target_company">
        <center>
            <p t-attf-style="background-color: {{ bar_color }}; position: relative; display: block; color: white; font-weight: bold;">
                <t t-esc="target_company.name"/>
            </p>
        </center>
    </div>

</xpath>

</data>

While doing a v18 upgrade this is what I am settling with for v18

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id='navbar_web_layout_comp_color' inherit_id='web.layout'>
        <data inherit_id="web.layout">
            <xpath expr="//body" position="inside">
                <t t-set="color" t-value='request.env["ir.config_parameter"].sudo().get_param("custom_icp.nav_bar_color") if request else False' />
                <t t-if='color'>
                    <t t-set="text_color">
                        <t t-set="r" t-value="int(color[1:3], 16)"/>
                        <t t-set="g" t-value="int(color[3:5], 16)"/>
                        <t t-set="b" t-value="int(color[5:7], 16)"/>
                        <t t-set="brightness" t-value="(r * 299 + g * 587 + b * 114) / 1000"/>
                        <t t-set="text_color_val" t-value="'white' if brightness &lt; 128 else 'black'"/>
                    </t>
                    <style>
                        .o_main_navbar {
                            background-color: <t t-esc="color"/> !important;
                            background: linear-gradient(45deg, <t t-esc="color"/>, <t t-esc="color"/>);
                            color: <t t-esc="text_color_val"/> !important;
                        }
                        .o_main_navbar *,
                        .o_main_navbar .breadcrumb-item,
                        .o_main_navbar .o_menu_toggle,
                        .o_main_navbar .o_user_menu,
                        .o_main_navbar a,
                        .o_main_navbar .o_MessagingMenu,
                        .o_main_navbar .o_debug_manager,
                        .o_main_navbar .o_menu_sections,
                        .o_main_navbar .o_menu_brand,
                        .o_main_navbar .o_switch_company_menu,                    
                        .o_main_navbar .dropdown-toggle {
                            color: <t t-esc="text_color_val"/> !important;
                        }
                        .o_main_navbar button,
                        .o_main_navbar .o_nav_entry,
                        .o_main_navbar .btn,
                        .o_main_navbar .btn-secondary,
                        .o_main_navbar .badge,
                        .o_main_navbar .dropdown-menu .dropdown-item {
                            color: <t t-esc="text_color_val"/> !important;
                            background-color: transparent !important;
                            border-color: rgba(255, 255, 255, 0.3);
                        }
                    </style>
                </t>
            </xpath>
        </data>
    </template>
</odoo>

Edit: Cant get screenshot to upload :(