Add fallbacks to CSS variables for older browsers

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};
+    }
+}