Walking through a local retail corridor last week I noticed how pervasive badly installed alarm systems are. installers needlessly disregard the thought that went into the design and asthetic of the space. I am not talking about spending more money. An effective but less intrusive alarm system typically does not cost more than a rushed installation that haphazardly slaps sensors onto a door.
There are a number of options for monitoring a door or window will maintaining the asthetic integrity of a space. The least invasive sensor is a resessed senor which is actually recessing into a door or window. This approach leaves virtually no visueal trace of the sensor however it may not be practical depending on the type of door.
The next best approach is a color matched "micro" or "slimline" sensor. These sensors or a fraction the size of a stanard sensor and generally perform just as well. However, the battery life is not as long so batteries may need to be replaced in a year or two rather than a few years.
Don't take it for granted than an alarm system is going to change the look of your doors and demand that it be as least invasive as possible.