Some of our built-ins just have holes that the shelve brackets fit into without the metal track bracket. In another room that had the bracket, I painted the same color as the built-ins

Probably the simplest idea would be to attach thin, white-painted flat boards, like a slat, about 1-1/2” tall, to the shelves so that the brackets are less noticeable from the front. This is NOT going to create an elegant shelving unit; you will need to build (or have a carpenter build) proper shelves, to avoid seeing those adjustable supports.

Can you take a picture further back. The shelves are very close together.
So I’d spread them apart a bit. And if you need to work with what you have already. I’d do some wooden corbels over those bracket things.
If you can go new look at ikea.

Normally, the standards (brackets) would be installed on the same wall to support a narrower shelf, but this seems to work as long as the shelves are sturdy and not overloaded.

Remove metal brackets and strips.
Measure and determine where you want shelves.
Nail or screw 1” x 1.5” strips on walls perpendicular to the back wall.
Paint to match wall
Shelves will rest on the strips, be hardly noticeable.
Review YouTube videos for ideas on most secure installation.
Caveat - shelf height will no longer be adjustable.

Where is this ? What do you honestly have that needs to be displayed ? IMO shelving units like this are meant much more for books or storage in a n office so a bit of context is IMO a good idea.

Mounting pre-drilled boards (in place of the current metal strips) will allow you to use glass for the shelves - as shown in the first photo comment.

https://www.lowes.com/pd/96-in-L-x-0-75-in-H-x-15-75-in-D-Thermally-Fused-Laminate-Wall-Mounted-Shelving/1000068951?cm_mmc=shp--c--prd--dcr--google--lia--232--shelving--1000068951-_-0&placeholder=null&ds_rl=1286981&gclid=Cj0KCQjw1PSDBhDbARIsAPeTqrdREVARXaOpv-FeDfsW81nAST32xDKZCz0SGY5eMHQkoUv0oZO26GgaAjIHEALw_wcB&gclsrc=aw.ds

If you don’t mind the shelf heights to be permanent, you can fill the holes with spackle and wallpaper the back wall with something fun (Etsy has tons of peel-and-stick wallpapers). Then remount new shelving with interesting brackets that are permanent. You need to look for nicer ways to mount nicer shelves and “dress up” the cabinet itself.

Thank you everyone for your very thoughtful and helpful comments. The shelves are in a family room and will be housing just decorative objects,