1) What obstacles exist to have the CHDK menu colors, splash screen etc. look the same on all cameras I have for an out of the box CHDK?
DIGIC II, III, 4, 5 uses paletted overlay. The palette is 4-bit on DIGIC II and III, and usually 8-bit on DIGIC 4, 5. Some early DIGIC 4 cams may use 4-bit overlay, but I'm not certain.
From CHDK perspective, the palette is always 8-bit.
The available colors are chosen by the Canon firmware, different palettes are used depending on play/rec mode and whether a Canon menu is visible. On DIGIC II and III cams, one has to live with the firmware's colors. On DIGIC 4 and 5 cams, there are methods for injecting the common CHDK colors into an unused place of the 256-entry Canon palette. CHDK can look the same on those cameras (but only in plain playback and rec mode). However, implementing this is not trivial.
A common, artificial palette is used on DIGIC 6 and later ports.
would someone please teach me some of what I need to know to do it.
The easy way is to pick the closest colors in play and rec mode, using the CHDK palette viewer. Find platform/(camera)/platform_palette.c in source and change the numbers in the ply_colors and rec_colors arrays. Those numbers are palette indexes as shown in the palette viewer.
Recompile when done. You could try this method on the ixus105.