If the color setting is undefined in theme.tab, it inherits the color of the previously selected theme.
For example, the tile cursor color is reddish orange if undefined. If you choose the modern theme first, it will be that color, but the Modern theme does not specify that color, and the undefined color will be assigned at initialization. Black is specified for high contrast. Switch to high contrast and then back to the modern theme. The cursor remains black. It inherited the high contrast theme's color setting without being initialized.
This applies to all colors that can be set in the theme.
See also: https://forum.simutrans.com/index.php/topic,19033