There was a mix of technologies. Up to about the early 80s, instruments were lit by an "unfiltered" incandescent lamp at the back of the instrument housing, that reflected off a band of white paint around the top of the housing and screened by the bezel, like old Smiths gauges. After about that point they moved over to edge-lit screen-printed perspex backings, and continued that way until the current trend for glarey and unpleasant LCDs everywhere.