The issue is that colours and formatting are generally defined by the event type, not the mere existence of text. For this to work, mIRC would have to pass along that event type ($event, maybe?), but handling text based on $event would pretty much be the same thing as simply handling the event, just way messier.
So yes, for a case like "I want to add [CRAP] to the beginning of every line no matter what it is"-- yea, it's overly complex. However, that scenario is also an oversimplification of what themes actually are, not to mention that scenario is exceedingly rare (and a little pointless, subjectively speaking).
Would you mind elaborating on exactly what "minor change" you're looking to make? Perhaps there's an easier way to go about this...
