docs: add dark mode feature entry to features page

Author: j4y

features.html

@@ -44,6 +44,10 @@
 <li><b>Portable mode — Keep settings with the executable</b><br />
 Launch Color Cop with the <code>-portable</code> switch to store configuration files alongside the executable instead of in LocalAppData. Ideal for USB drives, VMs, and portable toolkits.</li>
 
+<li><b>Dark mode — Match Windows or choose your theme</b><br />
+Color Cop supports system, light, and dark themes. When set to <i>System</i>, the UI automatically follows your Windows theme; you can also force Light or Dark mode from the Options menu.
+</li>
+
 <li><b>Other minor features</b><br />
 • RGB Float mode with adjustable precision (Alt+[ / Alt+])<br />
 • Optional standard Windows crosshair cursor<br />