In the game menu you need to add a new button with this code:
SDL_SetWindowFullscreen( var_sdl_window, (SDL_GetWindowFlags(var_sdl_window) & (Uint32)SDL_WINDOW_FULLSCREEN ? 0 : SDL_WINDOW_FULLSCREEN));
And also remove this fragment from the HTML file:
<div class="gears-cont"><div class="gears" id="gearBtn"></div></div>