Add dark theme support

src/popup/popup.less

@@ -50,3 +50,22 @@ body {
 .info {
     white-space: nowrap;
 }
+
+@media (prefers-color-scheme: dark) {
+    body {
+        background: #414141;
+        outline: 1px solid #7f7f7f;
+        color: #c4c4c4;
+    }
+    .copy {
+        background: url("../popup/copy-dark.svg") no-repeat;
+    }
+}
+
+@media (prefers-color-scheme: light) {
+    body {
+        background: #f1f3f5;
+        outline: 1px solid #7f7f7f;
+        color: #343a40;
+    }
+}