What is considered the customary practice regarding men's dress outdoors, in order to be mindful of the environment and the call to Islam (daʿwah)? And is it permissible to designate a different attire for prayer (such as a thobe or covering the head) if the general custom is to wear trousers and not cover the head, especially for the imam?

Wearing trousers for men is permissible as long as they are not excessively tight. It does not constitute imitation of disbelievers unless it is specific to them. Praying in tight trousers is disliked by some scholars and permissible by others. It is better to pray in loose garments to avoid disagreement. As for uncovering the head during prayer, it is permissible and does not affect its validity, because covering the head is a custom that varies from place to place. In summary, these matters are broad, and it is permissible for the Imam or the worshipper to wear trousers or uncover the head as long as there is a legitimate or da'wah-related benefit. Whoever wishes to cover his head or wear a thobe, that is better.