Try doing the following in a new module or in an already existing custom module:
Create a new css file under the file structure: your_module/static/src/css/your_style_sheet.css
Create a new XML file under the file structure: your_module/views/css_loader.xml
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',
],