Converting from sass to nested vanilla css with layers (#147)

assets/css/base/_document.css

@@ -0,0 +1,22 @@
+/* Document */
+
+@layer base {
+  html {
+    /* Change the default font family in all browsers. */
+    font-family: var(--ff-body);
+
+    /* Set base font size in percent. */
+    font-size: calc(var(--base-font-size) / 16 * 100%);
+
+    /* Set text colour. */
+    color: var(--color-text);
+  }
+
+  body {
+    /* Correct the line height in all browsers. */
+    line-height: var(--base-line-height);
+
+    /* Accent colour for forms. */
+    accent-color: var(--color-accent);
+  }
+}