No design changes are mandatory, no matter what the SE people say. If we write some Javascript code ourselves, we can make arbitrary customizations to the UI.

The MO foundation has an agreement with Stack Exchange that allows us to inject arbitrary javascript in the page, as long as "it does not compromise the technical integrity of the network".

Stack Exchange acknowledges that moderators may wish to make reasonable adjustments to the operation of MathOvertlow 2.0 by means of extra clientside JavaScript. Thus, moderators shall be permitted to submit additional JavaScript to Stack Exchange which, if it does not compromise the technical integrity of the network, will be inserted into the footer, allowing some reasonable modification of the site that is specific to MathOverflow 2.0. MathOverflow and you acknowledge and agree that the Stack Exchange 2.0 HTML is changing all the time, and accordingly, such JavaScript will need to be actively maintained by the moderators or it may stop working. It is understood that this script maintenance will he the sole responsibility of MathOverflow (including moderators) and not of Stack Exchange.

So if we don't like the new theme and Stack Exchange does not wish to invest any resources to make it better (as it is now, if I understand correctly), then we can roll out basically any arbitrary change for MO.