Hello,

You can do that by adding css files to the path then assign css classes to the html in your report

/static/src/css

Then You call the css file.

There are two main assets,frontend and backend assets.

Frontend assets are used for website while backend assets are used for the system backend, and since you want to use it in the report which i will assume will be in the backend you will inherit the assets_backend.

Calling the asset might differ from one version to another

If you are using odoo15 or later you call the css in the manifest by adding assets field

'assets': {
    'web.assets_backend': [
        'your_module_name/static/src/css/first_css_file.css',
	​	​'your_module_name/static/src/css/second_css_file.css',
    ],

If you are using odoo 14 or older version

You call the css in an xml view

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