Hi there,
Thanks for reaching out.
In reference to your query here, could you try out the following CSS code under the Appearance>Customize>Additional CSS section:
Well, you have a not easy problem here, most probably your theme i hijacked by malicious code, you need to check everything in your theme and plugins and clear them, the easiest way is to update WordPress, themes, and plugins.
this problem needs attention and professional investigation
thank you for responding. I tried deactivating plugins one by one and I feel I might have pinpointed a plugin that was causing the problem.
I was using a plugin: Banner Management for WooCommerce that I deactivated and the malware banner disappeared. Seems to work for now. Will keep you guys informed if I encounter this again.