init media.css

- combine all media queries
- name zmedia to conacat at last
- change some max-width values to group them

assets/css/theme-vars.css

@@ -17,12 +17,6 @@
     --border: #eee;
 }
 
-@media screen and (max-width:600px) {
-    :root {
-        --gap: 14px
-    }
-}
-
 .dark {
     --theme: #1d1e20;
     --entry: #2e2e33;