Add fallbacks to CSS variables for older browsers

assets/scss/code.scss

@@ -1,21 +1,5 @@
 @import "variables.scss";
 
-$code-color-lineno: grey;
-
-:root {
-    --code-color-keyword: black;
-    --code-color-type: black;
-    --code-color-comment: grey;
-}
-
-@media (prefers-color-scheme: dark) {
-    :root {
-        --code-color-keyword: #{darken(white, 40%)};
-        --code-color-type: #{darken(white, 40%)};
-        --code-color-comment: grey;
-    }
-}
-
 .highlight-label {
     padding: 0.25rem 0.5rem 0.25rem 0.5rem;
     border: $code-border;
@@ -32,7 +16,7 @@ $code-color-lineno: grey;
 
 code {
   font-family: $font-code;
-  background-color: var(--code-color);
+  @include var(background-color, code-color);
   border: $code-border;
   padding: 0 0.25rem 0 0.25rem;
 }
@@ -48,7 +32,7 @@ pre code {
     .lntable {
         border-spacing: 0;
         padding: 0.5rem 0 0.5rem 0;
-        background-color: var(--code-color);
+        @include var(background-color, code-color);
         border-radius: 0;
         border: $code-border;
         display: block;
@@ -80,7 +64,7 @@ pre code {
     .lnt {
         display: block;
         padding: 0 1rem 0 1rem;
-        color: $code-color-lineno;
+        color: $code-lineno-color;
     }
 
     .hl {
@@ -95,14 +79,14 @@ pre code {
 
 .kr, .k {
     font-weight: bold;
-    color: var(--code-color-keyword);
+    @include var(color, code-keyword-color);
 }
 
 .kt {
     font-weight: bold;
-    color: var(--code-color-type);
+    @include var(color, code-type-color);
 }
 
 .c, .c1 {
-    color: var(--code-color-comment);
+    color: $code-comment-color;
 }

assets/scss/margin.scss

@@ -1,10 +1,6 @@
 @import "variables.scss";
 @import "mixins.scss";
 
-$margin-width: 30rem;
-$margin-inner-offset: 0.5rem;
-$margin-outer-offset: 1rem;
-
 @mixin below-two-margins {
     @media screen and
         (max-width: $container-width-threshold +

assets/scss/modes.scss

@@ -0,0 +1,21 @@
+:root {
+    @each $varName, $varDefault in $css-vars {
+        --#{$varName}: #{$varDefault};
+    }
+}
+
+@media (prefers-color-scheme: dark) {
+    :root {
+        --background-color: #{$background-color-dark};
+        --text-color: white;
+
+        --warning-background-color: #{$warning-background-color-dark};
+        --warning-border-color: #{$warning-border-color-dark};
+
+        --target-background-color: #{$target-background-color-dark};
+
+        --code-color: #{$code-color-dark};
+        --code-keyword-color: #{$code-token-color-dark};
+        --code-type-color: #{$code-token-color-dark};
+    }
+}

assets/scss/sidenotes.scss

@@ -2,9 +2,6 @@
 @import "mixins.scss";
 @import "margin.scss";
 
-$sidenote-padding: 1rem;
-$sidenote-highlight-border-width: .2rem;
-
 .sidenote {
     &:hover {
         .sidenote-label {

assets/scss/style.scss

@@ -1,10 +1,11 @@
 @import "variables.scss";
 @import "mixins.scss";
 @import "toc.scss";
+@import "modes.scss";
 
 body {
-  background-color: var(--background-color);
+  @include var(background-color, background-color);
-  color: var(--text-color);
+  @include var(color, text-color);
   font-family: $font-body;
   font-size: 1.0rem;
   line-height: 1.5;
@@ -24,7 +25,7 @@ h1, h2, h3, h4, h5, h6 {
   text-align: center;
 
   &:target {
-    background-color: var(--target-background-color);
+    @include var(background-color, target-background-color);
     border-radius: 1rem;
   }
 
@@ -90,7 +91,7 @@ nav {
   a {
     padding: 0.25rem 0.75rem 0.25rem .75rem;
     text-decoration: none;
-    color: var(--text-color);
+    @include var(color, text-color);
     display: inline-block;
     border-bottom: none;
     white-space: nowrap;
@@ -130,7 +131,7 @@ h6 {
 }
 
 a {
-  color: var(--text-color);
+  @include var(color, text-color);
   text-decoration: none;
   border-bottom: .2rem solid $primary-color;
   transition: color 0.25s;
@@ -272,8 +273,8 @@ figure {
 .warning {
     @include bordered-block;
     padding: 0.5rem;
-    background-color: var(--warning-background-color);
+    @include var(background-color, warning-background-color);
-    border-color: var(--warning-border-color);
+    @include var(border-color, warning-border-color);
 }
 
 .feather {

assets/scss/toc.scss

@@ -2,8 +2,6 @@
 @import "mixins.scss";
 @import "margin.scss";
 
-$toc-border-color: $code-border-color;
-
 .table-of-contents {
     @include margin-content;
     @include margin-content-left;
@@ -45,7 +43,7 @@ $toc-border-color: $code-border-color;
     div.wrapper {
         @include bordered-block;
         padding: 1rem;
-        background-color: var(--code-color);
+        @include var(background-color, code-color);
         border-color: $toc-border-color;
         box-sizing: border-box;
         max-width: 100%;

assets/scss/variables.scss

@@ -1,15 +1,13 @@
 $container-width: 45rem;
 $container-min-padding: 1rem;
 $container-width-threshold: $container-width + 2 * $container-min-padding;
-$standard-border-width: .075rem;
 
 $primary-color: #36e281;
 $border-color: #bfbfbf;
 $background-color: white;
 $background-color-dark: #1b1d1f;
-$code-color: #f0f0f0;
+$standard-border-width: .075rem;
-$code-color-dark: lighten($background-color-dark, 10%);
+$standard-border: $standard-border-width solid $border-color;
-$code-border-color: darken($code-color, 10%);
 
 $font-heading: "Lora", serif;
 $font-body: "Raleway", serif;
@@ -23,25 +21,40 @@ $warning-border-color-dark: $warning-background-color;
 $target-background-color: lighten(yellow, 30%);
 $target-background-color-dark: #55511c;
 
-$standard-border: $standard-border-width solid $border-color;
+$code-color: #f0f0f0;
+$code-color-dark: lighten($background-color-dark, 10%);
+$code-token-color: black;
+$code-token-color-dark: darken(white, 40%);
+$code-border-color: darken($code-color, 10%);
+$code-lineno-color: grey;
+$code-comment-color: grey;
 $code-border: $standard-border-width solid $code-border-color;
 
-:root {
+$sidenote-padding: 1rem;
-    --background-color: #{$background-color};
+$sidenote-highlight-border-width: .2rem;
-    --text-color: black;
-    --code-color: #{$code-color};
-    --warning-background-color: #{$warning-background-color};
-    --warning-border-color: #{$warning-border-color};
-    --target-background-color: #{$target-background-color};
-}
 
-@media (prefers-color-scheme: dark) {
+$margin-width: 30rem;
-    :root {
+$margin-inner-offset: 0.5rem;
-        --background-color: #{$background-color-dark};
+$margin-outer-offset: 1rem;
-        --text-color: white;
+
-        --code-color: #{$code-color-dark};
+$toc-border-color: $code-border-color;
-        --warning-background-color: #{$warning-background-color-dark};
+
-        --warning-border-color: #{$warning-border-color-dark};
+// Default values for CSS variables for fallback.
-        --target-background-color: #{$target-background-color-dark};
+$css-vars: (
-    }
+    background-color: $background-color,
+    text-color: black,
+
+    warning-background-color: $warning-background-color,
+    warning-border-color: $warning-border-color,
+
+    target-background-color: $target-background-color,
+
+    code-color: $code-color,
+    code-keyword-color: $code-token-color,
+    code-type-color: $code-token-color,
+);
+
+@mixin var($property, $varName) {
+    #{$property}: map-get($css-vars, $varName);
+    #{$property}: var(--#{$varName});
 }