WIP on theme admin page (#20)

src/admin-theme/_theme-list-columns.liquid

@@ -0,0 +1,3 @@
+{%- assign theme_col = "col-12 col-md-6" -%}
+{%- assign slug_col  = "d-none d-md-block col-md-3" -%}
+{%- assign tmpl_col  = "d-none d-md-block col-md-3" -%}