Fixes

packages/create-docusaurus/templates/classic/src/css/custom.css

@@ -14,6 +14,7 @@
   --ifm-color-primary-lighter: #359962;
   --ifm-color-primary-lightest: #3cad6e;
   --ifm-code-font-size: 95%;
+  --docusaurus-highlighted-code-line-bg: rgba(0, 0, 0, 0.1);
 }
 
 /* For readability concerns, you should choose a lighter palette in dark mode. */
@@ -25,15 +26,5 @@
   --ifm-color-primary-light: #29d5b0;
   --ifm-color-primary-lighter: #32d8b4;
   --ifm-color-primary-lightest: #4fddbf;
-}
-
-.docusaurus-highlight-code-line {
-  background-color: rgba(0, 0, 0, 0.1);
-  display: block;
-  margin: 0 calc(-1 * var(--ifm-pre-padding));
-  padding: 0 var(--ifm-pre-padding);
-}
-
-[data-theme='dark'] .docusaurus-highlight-code-line {
-  background-color: rgba(0, 0, 0, 0.3);
+  --docusaurus-highlighted-code-line-bg: rgba(0, 0, 0, 0.3);
 }

packages/create-docusaurus/templates/facebook/src/css/custom.css

@@ -35,10 +35,3 @@
   --ifm-color-primary-lighter: #32d8b4;
   --ifm-color-primary-lightest: #4fddbf;
 }
-
-.docusaurus-highlight-code-line {
-  background-color: rgb(72, 77, 91);
-  display: block;
-  margin: 0 calc(-1 * var(--ifm-pre-padding));
-  padding: 0 var(--ifm-pre-padding);
-}

packages/docusaurus-theme-classic/src/theme/CodeBlock/Container/index.tsx

@@ -28,7 +28,7 @@ export default function CodeBlockContainer<T extends 'div' | 'pre'>({
       className={clsx(
         props.className,
         styles.codeBlockContainer,
-        ThemeClassNames.common.codeBlock,
+        ThemeClassNames.common.codeBlock.container,
       )}
     />
   );

packages/docusaurus-theme-classic/src/theme/CodeBlock/Line/index.tsx

@@ -6,6 +6,8 @@
  */
 
 import React from 'react';
+import clsx from 'clsx';
+import {ThemeClassNames} from '@docusaurus/theme-common';
 import type {Props} from '@theme/CodeBlock/Line';
 import styles from './styles.module.css';
 
@@ -26,7 +28,11 @@ export default function CodeBlockLine({
   });
 
   if (highlight) {
-    lineProps.className += ' docusaurus-highlight-code-line';
+    lineProps.className = clsx(
+      lineProps.className,
+      styles.highlightedCodeLine,
+      ThemeClassNames.common.codeBlock.highlightedCodeLine,
+    );
   }
 
   const lineTokens = line.map((token, key) => (

packages/docusaurus-theme-classic/src/theme/CodeBlock/Line/styles.module.css

@@ -5,6 +5,23 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+/* Intentionally has zero specificity, so that to be able to override
+the background in custom CSS file due bug https://github.com/facebook/docusaurus/issues/3678 */
+:where(:root) {
+  --docusaurus-highlighted-code-line-bg: rgb(72 77 91);
+}
+
+:where([data-theme='dark']) {
+  --docusaurus-highlighted-code-line-bg: rgb(100 100 100);
+}
+
+.highlightedCodeLine {
+  background-color: var(--docusaurus-highlighted-code-line-bg);
+  display: block;
+  margin: 0 calc(-1 * var(--ifm-pre-padding));
+  padding: 0 var(--ifm-pre-padding);
+}
+
 .codeLine {
   display: table-row;
   counter-increment: line-count;
@@ -25,7 +42,7 @@
   opacity: 0.4;
 }
 
-:global(.docusaurus-highlight-code-line) .codeLineNumber::before {
+.highlightedCodeLine .codeLineNumber::before {
   opacity: 0.8;
 }
 

packages/docusaurus-theme-common/src/utils/ThemeClassNames.ts

@@ -39,7 +39,10 @@ export const ThemeClassNames = {
     editThisPage: 'theme-edit-this-page',
     lastUpdated: 'theme-last-updated',
     backToTopButton: 'theme-back-to-top-button',
-    codeBlock: 'theme-code-block',
+    codeBlock: {
+      container: 'theme-code-block',
+      highlightedCodeLine: 'theme-code-block-highlighted-line',
+    },
   },
   layout: {
     // TODO add other stable classNames here

website/docs/guides/markdown-features/markdown-features-code-blocks.mdx

@@ -198,7 +198,7 @@ Supported commenting syntax:
 
 We will do our best to infer which set of comment styles to use based on the language, and default to allowing _all_ comment styles. If there's a comment style that is not currently supported, we are open to adding them! Pull requests welcome.
 
-You can also set your own background color for highlighted code line in your `src/css/custom.css` which will better fit to your selected syntax highlighting theme. The color given below works for the default highlighting theme (Palenight), so if you are using another theme, you will have to tweak the color accordingly.
+You can set your own background color for highlighted code line in your `src/css/custom.css` which will better fit to your selected syntax highlighting theme. The color given below works for the default highlighting theme (Palenight), so if you are using another theme, you will have to tweak the color accordingly.
 
 ```css title="/src/css/custom.css"
 :root {
@@ -212,6 +212,8 @@ You can also set your own background color for highlighted code line in your `sr
 }
 ```
 
+If you also need to style the highlighted code line in some other way, you can target on `theme-code-block-highlighted-line` CSS class.
+
 ### Highlighting with metadata string {#highlighting-with-metadata-string}
 
 You can also specify highlighted line ranges within the language meta string (leave a space after the language). To highlight multiple lines, separate the line numbers by commas or use the range syntax to select a chunk of lines. This feature uses the `parse-number-range` library and you can find [more syntax](https://www.npmjs.com/package/parse-numeric-range) on their project details.