Yup, overlaying seems very easy possible. I only have to add a new base layer in the looknfeel file that always displays the normal image.
This means, we only need the following
- one basic button
- one hover effect
- one disabled effect
- one puhshed effect
- raw content for each button (raw meaning ONLY the content with transparent bg, not the normal button frame or bg)
This way I only have to add a default layer (displaying the empty basic button), a content layer (raw content for each button specified in the layout) and an effect layer (containing a special, state-dependent effect image or nothing in case of normal state).
This will make creating the buttons way more easy.
And also changing specific parts will be as easy as posible this way.
With this we can already start building up a nice GUI without a clear decision on the final style. Because with this we have to change only ONE image to change ALL buttons to the new style.