The text is hard to read due to a shadow behind the letters

We use Optin forms on our website: https://www.detekta.si/ You can find it at the bottom of each page. as you can see, it is very hard to read text as the font appears with a shadow behind every single letter. In the plugin setup panel in WordPress backend the text is easy to read, clear and without any shadow. I dont find any style setting which could help us.

Kindly ask you for your support. BR,
Mateja