[BASIC] Update to the BASIC Programming chapter that fills all the "holes". (#281)

* WIP: Completed to the IF...THEN statement.

* WIP: Completed to the NEXT statement.

* WIP: Added images for the PRINT command.

* WIP: Added more graphics and content for the PRINT statement section

* WIP: Added more images for PRINT, completed to RIGHT$.

* WIP: Completed to STR$.

* WIP - Started adding internal links to keyword references.

* Change references of the Logo key to the ALT key.

* WIP: All remaining BASIC keywords documented.

* Moved CSS styling to the correct place for it.

* WIP: Edited down to CONT.

* WIP: All keyword documentation is complete.  Added new graphics to replace two of the quote mode examples.

* Fixed minor capitalization issue.

* add EOF newline

---------

Co-authored-by: MooingLemur <mooinglemur@users.noreply.github.com>

Author: geneb

.gh/markdown.css

@@ -133,3 +133,43 @@ table tr {
 table tr:nth-child(2n) {
 	background-color: whitesmoke;
 }
+
+/* 
+ * The mark and marksq styles will change the colors of the Markdown highlight feature based on 
+ * the viewer's dark mode settings.  Light mode will get white text on a black background,
+ * and dark mode should get black text on a white background.
+ * It also adds a border radius to the background to give the highlighted items a more
+ * "key-like" look.
+ */
+
+mark { /* gives a rounded "key" look to highlighted items. */
+  background-color: #000000;
+  color: #f5f5f5;
+  border-radius: 5px; 
+  padding: 0 4px; 
+}
+
+/* Dark Mode */
+@media (prefers-color-scheme: dark) {
+  mark {
+    background-color: #f5f5f5;
+    color: #000000;
+    border-radius: 5px; 
+    padding: 0 4px; 
+  }
+}
+
+marksq { /* used to letters with a square background */
+  background-color: #000000;
+  color: #f5f5f5;
+  padding: 0 4px; 
+}
+
+/* Dark Mode */
+@media (prefers-color-scheme: dark) {
+  marksq {
+    background-color: #f5f5f5;
+    color: #000000;
+    padding: 0 4px; 
+  }
+}