When skinning gameplay elements, complete sets of elements must be skinned. For instance, if you skin hitcircles, you must include a hitcircle.png, hitcircleoverlay.png, approachcircle.png, and a slider border color (this can be done by adding the line "SliderBorder: 255,255,255" (using RGB values) under the [Colours] section of each .osu file). The same logic applies to hitbursts, cursors, numbers, and anything else of this nature. If you didn't intend to skin an element in the set, then using the templateskin's version is okay. Setting a SkinPreference is also acceptable (e.g. forcing the default skin).A preciser explanation should be:
When skinning gameplay elements, complete sets of elements must be skinned. For instance, if you skin hitcircles, you must include a hitcircle.png, hitcircleoverlay.png, approachcircle.png, and the same slider border color as the hitcircle border color (this can be done by adding the line "SliderBorder: R,G,B" (using RGB decimal values) under the [Colours] section of each .osu file). The same logic applies to hitbursts, cursors, numbers, and anything else of this nature. If you didn't intend to skin an element in the set, then using the templateskin's version is okay. Setting a SkinPreference is also acceptable (e.g. forcing the default skin).