Let us be clear about what this ticket is about.
This ticket is not about redesigning the Configure Project page. Rather than being a bug caused by
JENKINS-68248, the effort to redesign the Configure Project page is a new feature at the same level as JENKINS-68248. Therefore it deserves its own top-level ticket. I have filed JENKINS-68282 for this purpose.
This ticket is about a bug caused by
JENKINS-68248; namely, that the bottom of the page lacks a border in the same way that the left and right of the page do. This does not seem intentional.
True, the redesign of the Configure Project page in
JENKINS-68282 may solve this bug (in which case this bug could be closed once JENKINS-68282 is landed), but let us not conflate the two: they are not equivalent in scope, nor are they equivalent in risk. Although this ticket is low in priority, a targeted bug fix for this ticket would have low risk (and therefore safe for a backport), while a redesign of the Configure Project page as in JENKINS-68282 is high risk (and therefore not likely a good candidate for backport).