As noted in T64177, when the background of toolbar icons is light, it's hard to see light grey icons. This patch makes them darker, specifically darker shade of the color of the geom icon, wherever applies.
To test, go to Blender Preferences, Themes in the sidebar, expand user interface, expand toolbar item. Bump up "inner" to put the dot towards the whiter side, over half of the length. Then reopen blender. Further work to do is to change it without a restart.
The function Color_Dark takes in uint *col = (void *)geom->colors, an array defined outside this function to store the RGBA info as long as col inside uses it, and a theme pointer bTheme *btheme = UI_GetTheme() used in icon_draw_size() of interface_icons.c.
btheme or later testtheme gives data about inner color, its brightness is calculated, the brightness of one cell ( triangle I guess) is also calculated. When both over 0.5f, RGBA → RGB + A → HSL and L-inversion → RGB → RGBA. This is repeated for the three vertices, i.e. col, col + 1, col + 2.