That's actually not really part of your HUD.
In addition to maintaining my HUD, I sometimes make some backgrounds. The files you're looking for are in the materials -> console -> characters folder. Most HUD's will not come with this - mine does because I use a custom BG that removes said characters - so you'll need to extract them via GCFScape. The .vtf's and .vmt's you'll need to edit correspond to the ones with the "fwk" or "mvm" suffix as those are not restricted to special events like Halloween or Smissmas.
I would leave the .vmt's alone and just mess with the .vtf's as those are the raw images in valve texture format. Open it in VTFEdit and save out as a .png or whatever image format you want and make edits in Photoshop/Gimp/MS Paint/whatever and then save back out in the same format. Take note of the flagged checkboxes in GCFScape when you import one of the .vtf's as you'll need to recheck the same ones when exporting back out as a .vtf after your edits to make sure it has the same quality and that no mipmapping occurs.
For your edits, you literally just need to make the image totally transparent, so that's simple enough in any image editing program.
This is a pretty time consuming process, but it gets more streamlined as you go along.
Have fun!