Issue #2084401 by Tharna: Fixed Themes can't contain numbers in name.

core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php

@@ -166,8 +166,8 @@ public function isValid() {
       )));
     }
     // Check for illegal characters in breakpoint source.
-    if (preg_match('/[^a-z_]+/', $this->source)) {
-      throw new InvalidBreakpointSourceException(format_string("Invalid value '@source' for breakpoint source property. Breakpoint source property can only contain lowercase letters and underscores.", array('@source' => $this->source)));
+    if (preg_match('/[^0-9a-z_]+/', $this->source)) {
+      throw new InvalidBreakpointSourceException(format_string("Invalid value '@source' for breakpoint source property. Breakpoint source property can only contain lowercase alphanumeric characters and underscores.", array('@source' => $this->source)));
     }
     // Check for illegal characters in breakpoint names.
     if (preg_match('/[^0-9a-z_\-]/', $this->name)) {