Steam goldsrc/source engines have a built in VGUI editor. Open the dialog box you want to edit in game, then press Ctrl+Alt+Shift+B
.
But if you want to edit the colours/fonts/etc, edit the trackerscheme.res file (copy them from <half-life>/platform/resource/
to <half-life>/<yourmod>/resource/
).
There's probably already a tool out there that lets you edit this scheme file that I didn't know of.