Obviously, since you're talking about me as well as the rest of the members who have volunteered, I'm not the best person to answer this.
Initially, we're just developing a dark theme for teh community. We do this in a styling language called "CSS" (Cascading Style Sheets).
In CSS, there is basically no way to do anything malicious. You can only customise how things look.
My current dark theme is totally open source so anyone can look at what is behind it and you can even contribute: https://github.com/davwheat/giffgaff-flarum-dark-mode
In the future, if we start going more in-depth into the forums, I'm sure there will be peer review stages where people check each others' code for any issues, then likely a stage where some giffstaffers look over it, too.
giffgaff likely has checks planned on us of some description, whether it be a coding test or something else.
I'll let a giffstaffer/roxy give you a more concrete answer as this is more as a guide but rest assured: the chance of anything bad coming out of this is less than members putting all of their details into one of the common scam messages.