Is it permissible to take fashion ideas from the internet and have them custom-made by a regular tailor? And is this considered theft?

The fundamental principle is that copyrights and patents are legally protected, and it is not permissible to infringe upon them. However, there is no harm in taking images available on the internet for a tailor to sew a similar garment for you, as long as it is for personal use and not for commercial purposes. Some scholars have explicitly permitted this, even if the owner has declared the preservation of their rights, just as Sheikh Al-Uthaymeen issued a fatwa (definition) allowing the copying of audio tapes for personal use, not for commercial trade.