You would need to edit the track panel background image files in the theme. The theme is structured as a ZIP file so you can
-- copy it to. say, Default_ch
-- unZip it,
-- edit the tcp_bg.png, tcp_bgsel.png, mcp_bg.png and mcp_bgsel.png files in an Image Editor that support transparency,
-- zip it up again (make sure that the file extension is still .ReaperThemeZip
-- select it in Reaper.
Have a look at the WALTER's Themer's Guide
http://www.houseofwhitetie.com/reape...mers_guide.pdf