I can use without any problems a custom beamer theme by installing the beamerthememyfile.sty in the same directory as the .tex-file and invoke it successfully by:
This is just a guess, since I'm not a mac user, but try this.
In a terminal type:
tlmgr conf | grep "TEXMFHOME"
It should show you where your local (personal) tex tree is located, or should be located. For me (on linux), it gives:
TEXMFHOME=/home/username/texmf
Mac probably uses "Users" rather than "home", etc., and your actual username rather than "username", etc.
Navigate to that directory (create it if need be):
cd /home/username/texmf
(or whatever)
See if there's a subfolder tex (if not, create it), and then a subfolder of it, latex, then a subfolder of it, beamer, and them a subfolder of it, themes. So the final folder might be something like:
/home/username/texmf/tex/latex/beamer/themes/
Put your .sty file in there, and then run texhash afterwards. (On linux, you need to run "sudo texhash" to run as superuser, but I don't know what the equivalent is on Mac, if any.) I hope that would work. You'll have to try it and let us know.
But now the theme is located in a system folder of my system. The Idea behind my request was, to put the theme in a kind of a Latex-Project folder, like:
so that all tex files of my Papers access the Themes-Folder and I can copy/backup the "MyLatexFiles" and am on the save side, that I have saved all files, which are needed
Well, maybe I'll try it just for the heck of it. I'll report back.
REPORT: Yeah, the latter option seems to work, at least for me. \usetheme{..} is basically the same as \usepackage{beamertheme..}, so you can achieve the same effect with \usepackage{Foldername/beamerthemeName} where the .sty file for the theme is beamerthemeName.sty.