diff --git a/front_end/cm/README.md b/front_end/cm/README.md
new file mode 100644
index 0000000000..890defe552
--- /dev/null
+++ b/front_end/cm/README.md
@@ -0,0 +1,3 @@
+Treat `codemirror.css` like a rolled-in upstream dependency. Instead of changing it directly, consider changing `front_end/text_editor/cmdevtools.css` instead.
+
+[Issue #1076825](https://bugs.chromium.org/p/chromium/issues/detail?id=1076825) tracks the effort to use the upstream file more directly.
diff --git a/front_end/cm/codemirror.css b/front_end/cm/codemirror.css
index c95ec0a422..1dcac08195 100644
--- a/front_end/cm/codemirror.css
+++ b/front_end/cm/codemirror.css
@@ -191,8 +191,6 @@ div.CodeMirror-dragcursors {
 .cm-tab-wrap-hack:after { content: ''; }
 
 /* Help users use markselection to safely style text background */
-span.CodeMirror-selectedtext { background: none; }
-
-.-theme-with-dark-background span.CodeMirror-selectedtext {
-  background: #000;
+span.CodeMirror-selectedtext {
+  background: none;
 }
diff --git a/front_end/text_editor/cmdevtools.css b/front_end/text_editor/cmdevtools.css
index b01e194a1d..50ee2bc912 100644
--- a/front_end/text_editor/cmdevtools.css
+++ b/front_end/text_editor/cmdevtools.css
@@ -628,6 +628,11 @@ div.CodeMirror:focus-within span.CodeMirror-nonmatchingbracket {
   background-color: #454545;
 }
 
+.-theme-with-dark-background span.CodeMirror-selectedtext,
+:host-context(.-theme-with-dark-background) span.CodeMirror-selectedtext {
+  background: #000;
+}
+
 .CodeMirror .auto-complete-text {
   color: rgb(128 128 128);
 }