You can right-click in the toolbar/switchbar to pop up a menu for changing these settings. Toolbar buttons can use RGB Color 255,0,255 for transparency, the BMP must be of the same form as that in mIRC resources. It should be a 16 or 256 color BMP.
You can create bmp files with graphic editor programs like "paint" (start - all programs - accessories - paint)
Try to google for a tutorial and more details, good luck.