Given your assumption - very ! (in my opinion)
Reaper themes are defined using WALTER - a powerful, flexible definition languages, covering positions, dynamic resizing, conditional displays ...
To get an idea, have a look thorugh the WALTER Guide, downloadable from here:
http://forum.cockos.com/showthread.php?t=69767
Here's an example of WALTER, from the Guide:
Code:
clear tcp.*
set tcp.size [258 72]
set tcp.trackidx [1 3 20 16 0 0 0 0]
set tcp.foldercomp [2 3 20 20 0 0 0 0]
;meter, recmon, recmode row
set tcp.meter h>=66 [66 46 188 20 0 0 1 0] [0]
set tcp.recmon h>=66 [24 47 20 20 0 0 0 0] [0]
set tcp.recmode h>=66 [44 47 20 20 0 0 0 0] [0]
It sets the relative positions (the first 4 numbers in the [ ... ]) and absolute sizes (the second 4 numbers in the [ ... ]) for 5 display elements and only displays the last 3 if the TCP height is > 66 pixels.