Yes, an HOA has a duty to treat all members equally
You have my condolences. Dealing with an HOA board and/or fellow residents can be frustrating and exasperating. Unfortunately, as a practical matter, the high cost of legal advice often mean boards can ignore the law.
In Pennsylvania, as everywhere, the Board of an HOA has a fiduciary relation to the owners. Because they are a fiduciary the Board has a legal duty to run the HOA for the benefit of the owners, not themselves or their friends. As the PA statutes puts it, the Board "shall perform their duties...in good faith...in the best interests of the association..."
The duty to act "in good faith" is usually interpreted as including a duty to "treat all community members equally..." (For more on "fiduciary," "good faith" and "equal treatment" see here, here and here.)
Whether you are being treated equally depends on the particular facts of your situation, and on how exactly PA courts have interpreted PA law. While you might be able to find a good answer on line, chances are you'll need to talk to a PA attorney with experience in condo law.
You should also check your condo's governing documents. They may contain specific provisions about how decisions about additions such as lighting are to be handled.