Most games still allow keyboard input as well as mouse.
Using the TAB key, (above the CAPS LOCK key), will highlight buttons then pressing Enter will invoke it. Even though you can't see the Apply button, you can experiment with the TAB key and pressing Enter.
Other than that, a more dangerous way is to increase the resolution of your monitor display then running the game again. I've never tried that, and I see danger ahead, so that risk is yours to decide.
I changed the resolution of my homeworld 2 game and now i cant see half of the screen. Now because of that I can not see the apply button to change the resolution back. So i cant play the game now. I tried reinstalling it but the resolution was the same. So, is there a way to change these setting outside the game or reset them somehow? Havnt played this game in forever and was pumped to play it again. Any help will be very appreciated. Thank you