Is there a way to edit/create layout for the Tray tip info window?

Are there any available themes other than the included?

There are a few included that you can select in the "layouts" tab of the preferences dialog.
The traytip layouts can be edited just like the other layouts, the only difference is that they must include the letter "T" in the Type= line.

example from v1.1.x :

[full with buttons]
HBButtons = VolumeIcon Prev Stop Play Next OpenBrowser ExtraButtons Pref Quit
HBIndic = Sort 10Filter Queue 10Pos -Stars
HBTitle = LockSong _Title LabelsIcons
HBArtist = LockArtist _Artist
HBAlbum = LockAlbum _Album -Year
HBTime = Time _TimeBar
VBmain = HBButtons HBIndic _VBText -HBTime
VBText = 2HBTitle 2HBArtist 2HBAlbum
HBmain = _VBmain 2Filler1 -Cover(forceratio=1)
VolumeScroll = HBmain

or :

Window= borderwidth=0
HBmain = Play(size=menu) Next(size=menu) 2_TimeBar(text="%t by %a",minwidth=240) Cover
Name= _"Minimal tip"

I can't seem to find where the tooltip themes are stored.
The closest to the right place i can find is: /usr/share/gmusicbrowser but none of the files in there or /usr/share/gmusicbrowser/plugins seem to be tooltip themes

All the layouts (traytip and others) are in the /usr/share/gmusicbrowser/layouts file (I plan to break it into a few .layout files), you can put your own layouts in a .layout file in ~/.gmusicbrowser/layouts/

Thanks :)

Small problem, though:
On editing 'Shimmer_tray' to:

VBMain = HBTime Filler0 VBMain1
VBMain1 = HBLeft _HBRight
HBLeft = _Cover(forceratio=1,default=pix/elementary/no-cover.svg) VBText
VBText = 2HBAlbum HBYear _2HBTitle
HBButtons = Prev(size=small-toolbar) Play(size=small-toolbar) Next(size=small-toolbar)
HBTitle = LockSong _Title(font=11,interval=75)
HBArtist = LockArtist _Artist(font=9)
HBAlbum = LockAlbum _Album(font=9)
HBYear = Filler1 _Date(font=9,markup="%y") -Stars
HBTime = _TimeBar(minheight=7)
HBRight = HBButtons _2HBArtist
VSize0 = 3 Filler0
HSize0 = Filler1 LockArtist LockAlbum LockSong
HSize1 = Cover HBButtons

It now will not show at all.
Any ideas?

It shows up for me.

Can you launch it from a terminal and see if any error message are printed in the terminal ?
(to launch it from a terminal, simply open a terminal and type "gmusicbrowser")

Here's what i get (removed all the 'scanning folder' stuff)

[email protected]:~$ gmusicbrowser
GStreamer::Interfaces perl module not found -> visuals not available
Reading saved tags in /home/USER/.config/gmusicbrowser/gmbrc ...
@Fields=album_shuffle replaygain_album_gain skipcount file grouping compilation replaygain_track_peak modif lastskip replaygain_reference_level genre embedded_pictures label title replaygain_album_peak album_artist samprate filetype bitrate missing added channel title_or_file size rating album_artist_raw lastplay shuffle playcount embedded_lyrics replaygain_track_gain extension version track path length artist album comment disc year barefilename fullfilename_raw artists first_artist album_picture album_years version_or_empty uri artist_picture missingkey fullfilename at /usr/bin/../share/gmusicbrowser/ line 1046.
adding update code for artists at /usr/bin/../share/gmusicbrowser/ line 1165.
Reading saved tags in /home/USER/.config/gmusicbrowser/gmbrc ... done
These commands were not found : mpg123, flac123, mpg321, ogg123
=> these file types won't be played by the 123 output : mp3, oga, flac
SongArray_changed replace,
SongArray_changed replace,filter Filter=HASH(0xca87f40)
playing [Track]

And now the modified layout works...
Typical, eh!

Only took a few re-launches, it would seem.