Personally, if I were to make a Portal themed build.
The outside would be minimal / white with Aperture logo(s).
The inside would have water-cooling loops / lights being blue/orange.
You could also consider getting black stickers but outlining them in white, or use white stickers.
Was looking around for 3.5" to 5.25" bays, then thought - why not just use some cardboard! I managed to slip my SSD in there too, in 1 5.25" bay.
Not as ghetto as some of this stuff around here, but it works for me.
For anyone interested in temperatures: Max temps so far have been 34C.