Added dark mode for app.css

Added dark mode for app.css that will be used together with said file.

The CSS file only changes background and text color.

Author: JulianPrieber

css/app-dark.css

@@ -0,0 +1,4 @@
+* {
+  background-color: #292929 !important;
+  color: #FFFFFF !important;
+}