For that, I would use Meld or Kdiff/Kompare. Komodo Edit will do what you want as well but I prefer the way the others present it (side by side and very clear).
Komodo is good but rather heavy and less than rock-solid reliable. It is a good editor other than that and there is also a proprietary (expensive!) IDE version.
Are you working on something someone else did? Then check whether whoever worked on this previously has not used version control. Hidden or mysterious files/folders are an indicator (.git, .hg, __fossil__ ,.flckout are the ones I know). If so the changes have already been recorded.
*IF* you decide to give Geany a serious try: [wiki.geany.org
...] You can do the same trick with Kate, and probably more easily with Gedit (it may have a run external script option - not sure). It is really intended for compiling code, but all it actually does is run a command against a file. I (used to) use it for running Python and HTML linters/checkers.
Of course you can always do it with vim - You can do ANYTHING with vim provided you are prepared to memorize a million key combinations...
If you expect to spend a lot of time on this, then I would put it in version control. Check in the unmodified theme in first, overwrite with the modified theme, check that in, then diff the checkins and you can see what you want AND you are set up to track future changes.