I took the plunge. My five year old MagicMirror ground to a halt as far as Display resolution, Rotation and DIsplay control are concerned. Everything was working great on my Raspberry Pi 3B+. An unfortunate rpi-update broke everything. I struggled to restore Bullseye, and it was a shock to realise it is not easy to find the right official image.
I then tried Bookworm, the three issues continued to remain unsolved.
I then decided to install Trixie. Most of the transition of MM was smooth, ofcourse with a few hicks, but getting the display resolution and rotation was the most frustrating part.
I got it working finally. I have restored the Display resolution and the rotation succesfully.
I will shortly upload the entire process which was not only exhausting but of great learning value.
I need to recover the GPIO Notification Module which isnt working and resulting Motion control of monitor using RCWL0516.

@sdetweil Thank you for the info. However, I finally successfully migrated to Trixie with desired Monitor Resolution and Rotation. And i am using Raspberry Pi 3B+ with an Acer 27 Inch monitor.
With Trixie being launched only three weeks back, there is almost no solutions avaiable. I will shortly share my process of how it was achieved.
Thanks

Awesome. It really works. I had the same issue and it was really frustrating, until i stumbled upon this solution. Great my PIR-sensor module was revived. I just followed the steps mentioned and voila…it has come back to life. Thanks a ton, it is selfless gentlemen like yourself who keep this world going. Kudos.

@sdetweil
Bingo. All systems go…
Thanks once again.
I am doing the backup restore thing now.

@MMRIZE I am stumped. The changes to the color happened, but the resizing still not responding.

@sdetweil
Obviously i am doing something wrong.
I am unable to get to the style even when newsfeed element is highlighted. Or, the style is picking up from main.css and changing anything here will impact all styles.

@MMRIZE Finally ! Finally…
I disabled all modules, except newsfeed, then cleared out the custom.css configs and added a fresh config for newsfeed as suggested above and finally it worked.
So was the issue just because of the corrupt custom.css?
Anyways, this issue has enabled me to learn working with the developer console. I am glad.
Thank you @MMRIZE and thank you @sdetweil . Ever obliged.