[Question]How can I make my application to save custom options?

Hello!

I am working on an application and I want to add more "graphics" to it, like themes.
Well... The application comes with it's standard theme but I would like if people could chose they'r own from lets say an "options menu".

The theme will load but when I close the application and open it again it comes back with the standard theme.
Is there any way to save the "custom" theme so the application will start with it?(I was thinking about saving the "options" code in a .txt file and the application will load that file when it starts)

Thank You!