White text on dark backgrounds appears slightly blurred or glowing on my MacBook Air.

“High contrast text fringing/halo: […]The effect is subtle but noticeable, especially in dark mode or high-contrast situations. I’ve attached photos comparing both displays for reference. Has anyone else experienced this? Is this a known issue with the display or a software-related problem?”

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It’s probably just the pixelation of your first photo, that looks how it should be.

Age Should be of no Concern:

If your Mac is made for an update, then no it is never too old for it.

Try Updating to you Mac:

Have you updated your Mac, yet? If not. Backup and the proceed with dong so.

Important: Back up your Mac:

Prior to update your Mac, create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the update.