[nightpool]

Unfortunately, because these types of preferences (font size, dark/light mode theme) are "non-essential", you are required to inform users about them using a cookie banner, per EU ePrivacy directive (the one that predates the GDPR). So if you don't use a cookie banner in this case, you are not in compliance.

[gregopet]

That's not true. You can use those cookies, you just need to explain them somewhere on the site. No opt in required.

I talked with our then national information law official (funny fact, same person is currently president of our country), rule of thumb is if you're not using your users' personal data to pay for other people's services (e.g Google analytics) or putting actual personal data in them, you're generally fine without the banner.

Further, if you're a small shop or individual acting in good faith and somehow still violated the law, they will issue a warning first so you can fix the issue. Only the blatant violations by people who should've known better will get a fine instantly (that is the practice here, anyway, I assumed that was the agreement between EU information officers)