try conditional css and do not accept overrides

src/css/watermark.css

@@ -1,12 +1,17 @@
+@media screen and (max-width: 76.1875em) {
+    .md-nav__title[for=__drawer] {
+        background-color: #ff0000 !important;
+    }
+    }
+
+.md-header {
+    background-color: #ff0000;
+}
+
 .md-main__inner {
     background-image: url(../images/draft_watermark.png);
     background-repeat: no-repeat;
     background-attachment: fixed;
     background-size: contain, cover;
     background-position: center;
 }
-
- /* See: https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#custom-color-schemes */
-:root {
-    --md-primary-fg-color: #ff0000;
-}