toolbars are saved in gui.json, in your calibre config folder at %APPDATA%/calibre.
for instance, quoted from gui.json,
Code:
"action-layout-toolbar": [
"Add Books",
"Edit Metadata",
null,
"Convert Books",
"View",
null,
"Choose Library",
"Save To Disk",
"Connect Share",
null,
"Preferences",
null,
"Overdrive Link"
],