Since: 1.3.8

You can add overlay images to the theme which will be shown on some specific
conditions:

* custom_0 - will be displayed when df0 LED is active.
* custom_1 - will be displayed when df1 LED is active.
* custom_2 - will be displayed when df2 LED is active.
* custom_3 - will be displayed when df3 LED is active.
* custom_4 - will be displayed when a disk is inserted into df0
* custom_5 - will be displayed when a disk is inserted into df1
* custom_6 - will be displayed when a disk is inserted into df2
* custom_7 - will be displayed when a disk is inserted into df3

The images themselves should be named custom0.png, custom1.png, etc. and
put in the themes folder. You only have to provide images for the effects
you want to show. The images can have any dimensions.
