feat(keymap-upgrader): Highlight changes

Updated the keymap upgrader to highlight which lines it changed as well
as indicate when nothing needed to be upgraded.

Also adjusted the line highlight colors to be more readable in both
light and dark color schemes.

docs/src/css/custom.css

@@ -15,10 +15,15 @@
   --ifm-color-primary-lighter: #0280e3;
   --ifm-color-primary-lightest: #0690fc;
   --ifm-code-font-size: 95%;
+
+  --docusaurus-highlighted-code-line-bg: rgb(0 0 0 / 8%);
+}
+
+[data-theme="dark"] {
+  --docusaurus-highlighted-code-line-bg: rgb(255 255 255 / 8%);
 }
 
 .docusaurus-highlight-code-line {
-  background-color: rgb(72, 77, 91);
   display: block;
   margin: 0 calc(-1 * var(--ifm-pre-padding));
   padding: 0 var(--ifm-pre-padding);