Try doing the following in a new module or in an already existing custom module:

1. Create a new css file under the file structure: your_module/static/src/css/your_style_sheet.css

2. Create a new XML file under the file structure: your_module/views/css_loader.xml

3. Add a new line in your modules `__manifest__.py`'s data section `views/css_loader.xml`

Edit the `css_loader.xml` file to look like this:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
  <data>
    <!-- Load css File -->
    <template id="my_custom_css_asset" name="my_custom_css assets" inherit_id="web.assets_backend">
      <xpath expr="." position="inside">
        <link rel="stylesheet" href="/your_module/static/src/css/your_style_sheet.css"/>
      </xpath>
    </template>
  </data>
</odoo>

The `__manifest__.py` files data section should look like this after the edit:

'data': [
    'views/foo.xml',
    'views/ls_ui.xml',
    'views/bar.xml',
    ],

​