I have an MSI MAG x870e Tomahawk WiFi mobo, and the temperature LED is just, like, SO bright.
I don't see an obvious way of changing its brightness or turning it off in the BIOS, so I'm thinking of cutting a 1"x0.5" rectangle of ND-8 photo gel and electrical taping it to the light.
Why do you even need to see that light? I chose a solid metal case, simple as. Noise dampened, protects against electromagnetic interference and isn't prone to spontaneously exploding as many glass cases are.
Also double check for on off switches. I have found on MSI and ASROCK hardware they have an undocumented led powerswitch hidden somewhere on them sometimes.