Depends on the system. If it's a brand new one there will usually be a set of global firm standards that should be followe (i.e. Oracle, Linux, Java or whatever) but you can follow a process to get an exception. It's not great but as banks have 3,000+ systems of various types they need to keep some control of the technology stacks in use.

IT is still back office, but in the last couple of jobs where I have dealt with the Front Office directly no-one laughs at you. They can't do their job without technology.