@woofertv
As other people have said before:
Backup before you Flash!
From my experience the worst you can do is
soft brick your phone.
Worst Case Scenario:
1. Boot loop: constantly reboot
OR
2. No Boot: get stuck at bootanimation (boot up and boot animation runs forever, until you restart)
In both situations: Recovery and
Download mode are still available; If you have a nandroid you can restore to a working state from within CWM.
In extreme situations or you want a fresh install; Enter download mode and Re-flash with Heimdall/Odin then restore data from backups (Titanium Backup Root, MyBackup Root etc)
Other Issues you may encounter:
If the theme or apps it includes are incompatible...
3. App Force Close/ Launcher FC.
4. Wierd graphical issues (misaligned text, icons not showing up, misaligned buttons)
5. Loss or renaming/mismatch of menu items.
6. Missing System apps or System apps unable to start due to incompatibility. (Good example; Phone.apk / Contacts.apk)
7. ROM boots but does not allow launcher to start (black screen, but title bar visible)
8. Lock screen causing reboots.
9. Selecting specific options from Launcher menu causes reboot.
Best Case Scenario:
You install a compatible theme on your ROM - you will have an experience where the theme is changed as you expect, everything works and your phone looks much nicer
I have never had a theme I applied that could not 1. Enter Recovery 2. Enter Download.
If you can do at minimum enter download mode then your phone is not bricked.