From Wikipedia talk:Requests for comment/Rollback of Vector 2022 § Proposal for next steps following the closure of the Vector 2022 RfC § April 6th Response:

The modal should have extremely neutral wording, such as "Wide width / Narrow width" instead of "Unlimited width / Existing width" ("existing" is biased). And "Select wide width / Select narrow width" instead of "Revert to unlimited text width / Keep existing width" ("revert" and "existing" are biased). Then this unbiased data can be used to figure out what width readers prefer. Hope this helps. –Novem Linguae (talk) 22:56, 6 April 2023 (UTC)

I share Novem Linguae's concerns, and would also add that the blue and white buttons could also be a source of bias; many users are likely to just click whichever button seems likeliest to get the popup out of their way so that they can read the article, and will click the blue button without much reading (similarly to clicking an "accept all cookies" button).

Suggestions:

For color: randomize the button colors, and (if you can manage it) the order in which they are presented.
Try both this and tooltips as different treatments of a sample of the same population (logged-in or logged-out) for better understanding of preferences.
Try this as treatments for a sample of both logged-in and logged-out for contrasting understanding of those two populations.