... one could argue that the same is true for most other bittorrent clients out there, which have even less interface customization options than Tixati...
What is your suggestion: tread water together with also-rans or move onward?
Google for "progress bar image" — you will hardly find an indicator as boring as this one.
A nice GUI of any program makes your work with it more pleasant, thus more effective.
I'm guessing it uses the system colours, so to change them you'd have to change setting in your OS...
Are you serious? So, why the system progress bars are green or blue? The system doesn't use the system colours?