have specific styles @import the base styles

this clarifies the value of what was formerly "style.css" a bit, and
also opens the door for potential styles that don't inherit the base
styling at all

incorporealcms/static/css/dark.css

@@ -1,3 +1,5 @@
+@import '/static/css/base.css';
+
 html {
     color: #CCC;
 }

incorporealcms/static/css/light.css

@@ -1,3 +1,5 @@
+@import '/static/css/base.css';
+
 html {
     color: #222;
 }