Currently, if a preset requires a brush that doesn't exist, the preset docker shows that preset with a small icon in the bottom corner of the thumbnail. I believe the icon in question is called "edit-delete" (usually intended for when the user can delete something by clicking the icon). Depending on the system icon theme, this may be rendered as a red cross, a bin or any number of things that might represent deletion. I suggest replacing this with a custom icon could be:
a) Clearer to a user who hasn't come across it before.
b) Easier to explain in documentation (or on irc or wherever).
If the icon only appears when the required brush is missing, this could be an image of a brush in a 'no sign'.
If the icon can also mean that the preset's settings are missing/invalid, an icon relating to an error might be more appropriate.


Reproducible: Always

Steps to Reproduce:
1. Create a preset that refers to a brush that doesn't exist
2. Look at the preset in the Preset docker.
3. Try several different system icon themes.
Actual Results:  
The preset docker shows the preset marked with an icon that:
a) relates to deletion
b) varies between icon themes

Expected Results:  
The preset docker shows the preset marked with an icon that:
a) depicts something to do with missing brushes
b) is consistent across icon themes (probably by being a custom icon included as part of krita)