On Emacs Theme Precedence
I’ve written before about themes in Emacs, and in particular about using them for something other than just colors. And naturally, you can have more than one theme active at the same time. So what happens when you have a variable set in two active themes?
The list of enabled themes is the custom-enabled-themes variable. And rather intuitively, earlier ones override later ones. Normally, (load-theme 'mytheme) prepends mytheme to custom-enabled-themes, so the settings you just loaded take effect, and last until you load some other settings on top of them, which makes sense.
There’s one twist, though: the settings you change through the “Customize Emacs” system also count as a theme, at least in some ways. If you look at the property list for a symbol defined in two themes, you might see something like
(symbol-plist 'my-var) (theme-value ((user 'customized-value) (second-theme 'second-value) (first-theme 'first-value)) saved-value 'customized-value ...)
This tells us that two themes, first-theme and second-theme, have both defined my-var, and set it to first-value and second-value respectively. The symbol’s value is resolved in the order in which it appears here, so second-value overrides first-value because it was loaded later.
But there’s that third user entry, which comes first, and thus overrides the other values: that comes from (customize-variable 'my-var). I find this counterintuitive, since I think of the customization variables as the defaults, to be overridden by themes.
The solution I came up with this was to define my own theme, arensb, to hold all of the settings that might be overridden by other themes, and to load it first. Actually, I already had this theme lying around to store my favorite colors and faces, but themes can hold other values, so it made sense to reuse it.