Sorry, what?
If you want the select channel screen to be transparent, you have to edit he UI file for that - which probably is in ui_texture0.
If you want a background for the channels (so not seethrough/transparent), you have to remove the alpha channel for that area in the dds file.
Did you edit the files yourself or did you download them from somewhere?
You either need to remove the alpha channel from that specific part of the file, which is the part for the channel ui, or you need to save the .dds file as .dds and option 'Alpha Channel' - 'Alpha 1'. Using Photoshop here.