明るい and 暗い: light and atmosphere
Compare illumination with impressions of people or atmosphere, using context to distinguish a brightened expression from added light.
A well-lit space
窓が大きいので、昼は部屋が明るいです。 says that the room is bright during the day because it has a large window. 明るい, read あかるい, describes ample light.
廊下が暗くて、足元がよく見えません。 says that the corridor is dark and the speaker cannot see their footing well. 暗い, read くらい, describes insufficient light.
Cheerful character and gloomy atmosphere
彼女は明るくて、よく笑います。 says that she is cheerful and laughs often. It evaluates manner or character rather than literal light.
会議の後、部屋の雰囲気が暗くなりました。 says that the atmosphere in the room became gloomy after the meeting. It does not establish that the lights were turned off.
Expressions and areas of knowledge
知らせを聞いて、顔が明るくなりました。 can mean that someone's face lit up on hearing the news. Whether literal light changed depends on context.
この地域の事情に明るい人に聞きましょう。 proposes asking someone familiar with local circumstances. に明るい can mean knowledgeable in an area, not merely cheerful.
Must 地域の事情に明るい人 have a cheerful personality?
Show the answer and why
No. It means someone knowledgeable about local circumstances. Identify whether 明るい concerns light, character, or knowledge.