Oh you're talking about AdGuard VPN not solely the main AdGuard product. Definitely not ideal. It doesn't offer the same level of features as my current VPN who offers ad blocking anyway. Not to mention a few suspicious quotes from their website:
AdGuard VPN protocol uses the most secure and fast encryption algorithm to date – AES-256
From the very outset, we resolved to develop and deploy an in-house VPN protocol instead of picking a canned solution — that’d be too easy
We are going to make our protocol implementation publicly available in the future. Sadly, right now we don't have enough time to prepare the project
we collect data about how you interact with our services, how much traffic you’ve used, and for how long have you been using our services
ADGUARD SOFTWARE LIMITED is a company registered in Nicosia, Cyprus, registered office is at Klimentos 41-43, KLIMENTOS TOWER, Flat/Office 25, 1061, Nicosia, Cyprus and acts as the data controller when processing your data
Considering Cyprus telecommunications laws it doesn't seem like the safest place to headquarter a telecommunications privacy company.
CORRECTION: It was pointed out to me that I was confusing userStyles.css with userChrome.css. I'm not aware of anyway that styling the UI outside of the DOM could be directly detectable. Theoretically, if you could resize some UI elements that would change the viewport size and therefore be detectable. I'm not informed enough on userChrome.css to make that call however. Original comment is below.
Yes. As CSS styles can be queried, any differences can be used as an identifiable data point used to form a fingerprint.