Override the min-width for body from the alabaster theme

For some reason it sets min width to 450px which is too wide
for some mobile screens. Fixes #3116

docs/_static/custom.css

@@ -21,6 +21,10 @@ div.body p, div.body dd, div.body li, div.body blockquote {
     text-align: justify;
 }
 
+div.body {
+    min-width: 200px;
+}
+
 pre {
     white-space: pre-wrap;
 }