People put on a mask when they go to church. Sometimes you see them outside of church dealing with real life and they have forgotten to put on their countenance. Does that mean they are sinning?
I have many friends whose countenance brightened noticeably when they were freed from the need to put on that mask and decided to just be themselves. A TBM looking for signs that they were still fully orthodox might interpret changes in their clothing, hair length, facial hair, etc. as darkened countenance, whereas someone who is sincerely interested in their wellbeing, and who is willing to interact with an open mind will discover that they are genuinely more happy.
People often misinterpret disapproval of behaviors of church leaders, distorted history, and gaslighting as a darkened countenance. If you are pissed off to learn that your leaders are committing massive SEC and IRS fraud, you are likely to not wear a happy face when the topic comes up. That is perceived as a darkened countenance. When you learn that they are siding with the perpetrators of child sexual abuse rather than victims your countenance when their names are mentioned may not look bright. When you hear stories about Joseph Smith's loving devotion to Emma you might do this

instead of this

. Darkened countenance. When you hear these things about your leaders, history, and doctrine you are supposed to just keep doing this

so your everyone knows your countenance remains undimmed.
I just saw an entire movie about bright countenances.

“The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also.” -Mark Twain
Jesus: "The Kingdom of God is within you." The Buddha: "Be your own light."