Hi,

1-  Create a new css file (your_module/static/src/css/your_style_sheet.css)

2-  Create a XML file (your_module/views/css_loader.xml)

3- In css_loader.xml

<?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>

Then you can use class ="your_custom_class" attribute in your button definition in xml file.

Hope it helps

May it be a problem with the css file permissions? Please, check in the inspector if your css file is being loaded.