Add theme_slate.css. Brainstorming.

Will continue developing the themes idea later, just need to
get this out of my git commmit queue until I come back to it.

frontends/etiquette_flask/static/css/theme_slate.css

@@ -0,0 +1,8 @@
+:root
+{
+    --color_site_theme: #333;
+    --color_site_secondary: #3b4d5d;
+    --color_site_textfields: var(--color_site_secondary);
+    --color_text_normal: #efefef;
+    --color_text_link: #1edeff;
+}