Make sure the icon will be in svg format, and only a few kilobytes.
The icon should be 'fill' based (not 'stroke'). With this convention, we are able to color the icons according to the app-styles and view state. Do NOT add any color to the icons as we do not support it - it will be colored according to the app-styles.
In order to convert an icon from stroke base to fill base, most of the illustration apps support Expand/Outline Stroke function.
This function with the <add+> layers / merge layers function enable merging expanded stroke into one SVG path that can be colored.
An example of the converted icon is attached.