Yes and no. If I follow the logic, then currently it's something like this:

-- layout defines property $X
-- layout sets $X = 'blue' ---> default value
-- theme sets $X = 'green' ---> default value for theme
-- user sets $X = 'red' ---> overrides all of the above

Right? So if instead of setting $X in the theme, I define it in the layout like so:

-- layout defines property $X
-- layout defines property $Y
-- layout sets $X = 'blue' ---> default value
-- layout sets $Y = true
-- layout function sez:
... if $Y is true, $X = 'blue', else $X = $Z
-- theme sets $Y = false
-- theme sets $Z = green
-- user sets $X = 'red' ---> overrides all of the above by setting $Y as true and $X as red, theme value is ignored

... OR I could just say to hell with it and do it long-hand, because this is like spending twenty dollars to save ten cents, isn't it.