Is it permissible to take a small amount from the gratuity—which employees collect and divide equally—to compensate for theft committed by some colleagues, given the employer's refusal to intervene?

It is impermissible to make tips part of the salary due to their uncertainty. It is permissible for the worker to take them if the employer grants permission and the worker fulfills their duties. If employees agree to share tips, there is no harm in that, as this falls under the category of bodily partnership (shirkah al-abdan). It is not permissible for any of the partners to conceal anything they have taken, as this constitutes betrayal and unlawful appropriation of wealth. The obligation is to disclose everything you have received, or to cease participating with them. If they persist in dishonesty, then the partnership should be dissolved.