Override the main website template to inject your code, as follows:

1. Add a file called views/templates.xml and add an empty view override, as follows (don't forget to list the file in __manifest__.py):

<odoo> 
    <template id="assets_frontend" 
              inherit_id="web.assets_frontend"> 
        <xpath expr="." position="inside"> 
            <!-- points 2 & 3 go here /--> 
        </xpath> 
    </template> 
</odoo> 

2. Add the reference of the CSS and SCSS file, as follows:

<link href="/my_library/static/src/css/my_library.css" rel="stylesheet" type="text/css"/>
<link href="/my_library/static/src/scss/my_library.scss" rel="stylesheet" type="text/scss"/>

3. Add a reference to your JavaScript file, as follows:

<script src="/my_library/static/src/js/my_library.js" type="text/JavaScript" />

4. Add some CSS code to static/src/css/my_library.css, as follows:

body main {
    background: #b9ced8;
}

5. Add some SCSS code to static/src/scss/my_library.scss, as follows:

$my-bg-color: #1C2529;
$my-text-color: #D3F4FF;

nav.navbar {
    background-color: $my-bg-color !important;
    .navbar-nav .nav-link span{
        color: darken($my-text-color, 15);
        font-weight: 600;
    }
}

footer.o_footer {
    background-color: $my-bg-color !important;
    color: $my-text-color;
}

6. Add some JavaScript code to static/src/js/my_library.js as follows:

odoo.define('my_library', function (require) {
    var core = require('web.core');

    alert(core._t('Hello world'));
    return {
        // if you created functionality to export, add it here
    }
});

After updating your module, you should see that Odoo websites have custom colors in the menu, body, and footer, and a somewhat annoying Hello world popup on each page load, as shown in the following screenshot: