Tentatively add a dark mode

assets/scss/style.scss

@@ -3,6 +3,8 @@
 @import "toc.scss";
 
 body {
+  background-color: var(--background-color);
+  color: var(--text-color);
   font-family: $font-body;
   font-size: 1.0rem;
   line-height: 1.5;
@@ -22,7 +24,7 @@ h1, h2, h3, h4, h5, h6 {
   text-align: center;
 
   &:target {
-    background-color: lighten(yellow, 30%);
+    background-color: var(--target-background-color);
     border-radius: 1rem;
   }
 
@@ -88,7 +90,7 @@ nav {
   a {
     padding: 0.25rem 0.75rem 0.25rem .75rem;
     text-decoration: none;
-    color: black;
+    color: var(--text-color);
     display: inline-block;
     border-bottom: none;
     white-space: nowrap;
@@ -128,7 +130,7 @@ h6 {
 }
 
 a {
-  color: black;
+  color: var(--text-color);
   text-decoration: none;
   border-bottom: .2rem solid $primary-color;
   transition: color 0.25s;
@@ -139,7 +141,8 @@ a {
 }
 
 img {
-  max-width: 100%
+  max-width: 100%;
+  background-color: white;
 }
 
 table {
@@ -231,6 +234,7 @@ ul.post-list {
 
 figure {
     img {
+        border-radius: 0.5rem;
         max-width: 70%;
         display: block;
         margin: auto;
@@ -268,8 +272,8 @@ figure {
 .warning {
     @include bordered-block;
     padding: 0.5rem;
-    background-color: #ffee99;
-    border-color: #f5c827;
+    background-color: var(--warning-background-color);
+    border-color: var(--warning-border-color);
 }
 
 .feather {