The Theme system of Remote Desktop Manager is still in its infancy and will evolve in future releases. You can override most images used by Remote Desktop Manager simply by providing an image in the Theme folder, the only requirement is that the file uses a specific name. Most images come in pairs, a small one and a large one. You must override both to be consistent.
The Theme folder can be found in your local Configuration folder, under the Images folder.
It must be a standard *.ico file containing at least a 16*16 image. The name must be Trayicon.ico
Png file, 16*16 pixels.
Png file, 32*32 pixels.
The file name is standardized as: {EntryType}{Size}.png
For instance, for the Folder type, the image names are: GroupFolderLarge.png and GroupFolderSmall.png Another example is for RDP sessions, their name is RDPConfigured, resulting in SessionRDPConfiguredLarge.png and SessionRDPConfiguredSmall.png.
Please consult Image List to see a list of most images.