Second Update...  I did some testing...  and it appears that the "RUN PHP" in the actions section is executed before any of the page loads.   Putting in:

$theme = "Winter";
Echo $theme;

Prints "Winter" at the top of the page, however adding the echo line within the PHP section of the page header shows the variable is null.  so it seems to be something about the variable being set globally.  I have no idea how to overcome this limitation.  I really don't want to make a separate template for every "season" I want to have.   If anyone can tell me what I'm doing wrong, and how I can fix it.  I really would appreciate it

Thanks for getting back to me....    That worked perfectly....   I also found that using DEFINE works also, but I like your method better

define("THEME", "Winter");

Then in the Template

if (THEME != "") {
    $theme = THEME;
} else {
    $theme = "Default";
}