I have to agree with Hook: use the gapa program!
I just got the gamma program gapa.exe and set up various gammas eg 1.0 hotkey A, 1.3 hotkeyB, 1.4 C, etc remembering not to use any hot key that Histwar uses (D,S,I,U,W,P etc) to avoid disabling their function. This took about 1 minute. gapa remembers these settings so before launching Histwar i just double click on gapa.exe on my desktop, then launch Histwar. Whenever anything looks too dark just hit the appropriate hot key and the gamma changes; if it is too light then choose a different hotkey. The point is it runs dynamically so it is almost continuously variable as you play: simple! and it solves all those too dark to see properly problems.... There is no noticable loss of performance in histwar having gapa run in the background (and this on an old Geforce4 ti 4600 128Mb card where it would be most noticeable!).