Initial special behavior for unset theme variables

Very minimal, but shows how some special behavior can be defined for
themes beyond "use base value if not set".

tildes/scss/themes/_black.scss

@@ -2,8 +2,6 @@ $theme-black: (
   'foreground-primary': #ccc,
   'foreground-secondary': #888,
   'foreground-highlight': #ddd,
-  'foreground-middle': #aaa,
-  'foreground-extreme': #fff,
   'background-primary': #000,
   'background-secondary': #222,
   'background-input': #000,

tildes/scss/themes/_dark.scss

@@ -2,8 +2,6 @@ $theme-dark: (
   'foreground-primary': $fg-light,
   'foreground-secondary': $fg-darkest,
   'foreground-highlight': $fg-lightest,
-  'foreground-middle': #6e8186,
-  'foreground-extreme': #fff,
   'background-primary': $bg-darkest,
   'background-secondary': $bg-dark,
   'background-input': #001f27,

tildes/scss/themes/_light.scss

@@ -2,7 +2,6 @@ $theme-light: (
   'foreground-primary': $fg-dark,
   'foreground-secondary': $fg-lightest,
   'foreground-highlight': $fg-darkest,
-  'foreground-middle': #7c8e92,
   'background-primary': $bg-lightest,
   'background-secondary': $bg-light,
   'background-input': #fefbf1,

tildes/scss/themes/_theme_base.scss

@@ -652,15 +652,29 @@
 }
 
 @function init-theme($theme-map) {
-  @return map-merge($theme-base, $theme-map);
+  $theme: map-merge($theme-base, $theme-map);
+
+  // set $is-light as a bool for whether background-primary seems light or dark
+  $is-light: lightness(map-get($theme, 'background-primary')) > 50;
+
+  // foreground-middle: if not defined, mix foreground-primary and foreground-secondary
+  $foreground-middle: map-get($theme, 'foreground-middle');
+  $foreground-middle: mix(map-get($theme, 'foreground-primary'), map-get($theme, 'foreground-secondary')) !default;
+
+  // foreground-extreme: if not defined, use white on a dark background and black on a light one
+  $foreground-extreme: map-get($theme, 'foreground-extreme');
+  $foreground-extreme: if($is-light, #000, #fff) !default;
+
+  @return map-merge($theme, (
+    'foreground-extreme': $foreground-extreme,
+    'foreground-middle': $foreground-middle,
+  ));
 }
 
 $theme-base: (
   'foreground-primary': #333,
   'foreground-secondary': #999,
   'foreground-highlight': #222,
-  'foreground-middle': #666,
-  'foreground-extreme': #000,
   'background-primary': #fff,
   'background-secondary': #eee,
   'background-input': #f7f7f7,