Slight changes with test from Ecometer.

assets/scss/print.scss

@@ -0,0 +1,21 @@
+/**
+ * Omegamma - Hugo theme for ad5001.eu and related websites.
+ * Copyright (C) 2021  Ad5001
+ * 
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ * 
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+@media only print {
+    body * { display: none !important; }
+    body:after { content: "Don't waste paper!"; }
+}