I would suggest an Addressable RGB solution. Probably lighting strips. RGB, because she's bound to chop and change her mind as to the colour later on!
Best solution, IMO, is the Aquacomputer Farbwerk 360:
- It doesn't need a motherboard with either RGB header types, as it connects directly to the USB header. Powered by a single Molex.
- It comes with two strips, 30cm long with 15 LEDs each, which is enough to light the inside of most full tower cases. However, you can always buy more and daisy-chain them. The strips have an adhesive backing, but are also magnetic so you can change them around and relocate them.
- It has four channels, and each channel can handle up to 90 LEDs, for a total of 360.... which is **** loads of lights!!!
- It is controlled by the (free) Aquasuite software and has probably the widest range of patterns of all the RGB software (unless you have the weeks of time to give up when making them in iCUE). You can also have the strips do things based on all sorts of internal sensor data (temperatures, HDD activity, etc etc etc) from teh motherboard, HWInfo, AIDA64 and all that stuff.
Basic Farbwerk360 package costs about £35.
Extra strips are about £8. Extra daisy-chaining cables are a couple of quid, in 4cm, 10cm, 30cm or 50cm.
OCUK sometimes have these in stock, or you can order through Aqua Computer's own website (in German, so click the little British flag up top).
Failing that, another set I recommend, which has a lot of the same basic features, is the Cablemod range. Bit more expensive though and starter kit only has one strip.
https://www.overclockers.co.uk/cablemod-widebeam-magnetic-rgb-led-kit-60cm-30-leds-cm-13a-cm.html
https://www.overclockers.co.uk/cablemod-widebeam-magnetic-rgb-led-kit-30cm-15-leds-cm-139-cm.html