Make CodeMirror editor gutter background the same color of the

text background.
Put line numbers in the center of the gutter.

packages/codemirror/style/index.css

@@ -51,8 +51,8 @@ pre.CodeMirror-line {
   padding: 0 var(--jp-code-padding);
 }
 
-.CodeMirror-linenumbers {
-  padding: 0 4px 0 4px;
+.CodeMirror-linenumber {
+  padding: 0 8px;
 }
 
 .jp-CodeMirrorEditor-static {
@@ -90,11 +90,7 @@ pre.CodeMirror-line {
 
 .CodeMirror-gutters {
   border-right: 1px solid var(--jp-border-color2);
-  background-color: var(--jp-layout-color2);
-}
-
-.CodeMirror-gutter-wrapper {
-  margin-left: calc(-1 * var(--jp-code-padding));
+  background-color: var(--jp-layout-color0);
 }
 
 .jp-CollaboratorCursor {