[OSK] Icon buttons are not themed

On-Screen Keyboard has some icon buttons (like Caps Lock, Enter, and so forth...) which are not rendered with the current theme. The system normally never renders these buttons so you've to render them manually in the application's code. On NT 6.x (Vista+ and onwards) systems, the system does automatically render them.

I already have the means to fix this however I cannot push a PR right now as I'm currently busy with school. I created this ticket as a reminder in the future.