Give the .CodeMirror class no background so ours shows through.

IPython/frontend/html/notebook/static/css/style.min.css

@@ -914,7 +914,7 b' div.output_latex{text-align:left;}'
 div.text_cell{padding:5px 5px 5px 5px;}
 div.text_cell_input{color:#000000;border:1px solid #cfcfcf;border-radius:4px;background:#f7f7f7;}
 div.text_cell_render{outline:none;resize:none;width:inherit;border-style:none;padding:5px;color:#000000;}
-.CodeMirror{line-height:1.231;height:auto;}
+.CodeMirror{line-height:1.231;height:auto;background:none;}
 .CodeMirror-scroll{overflow-y:hidden;overflow-x:auto;}
 .ansiblack{color:#000000;}
 .ansired{color:darkred;}

IPython/frontend/html/notebook/static/less/notebook.less

@@ -368,6 +368,7 b' div.text_cell_render {'
 .CodeMirror {
     line-height: 1.231;  /* Changed from 1em to our global default */
 	height: auto;     /* Changed to auto to autogrow */
+	background: none;  /* Changed from white to allow our bg to show through */
 }
 
 .CodeMirror-scroll {