minor #34174 [TwigBundle][exception] Added missing css variable to hi…

…ghlight line in trace (lyrixx)

This PR was merged into the 4.3 branch.

Discussion
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[TwigBundle][exception] Added missing css variable to highlight line in trace

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

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To get the yellow background
![image](https://user-images.githubusercontent.com/408368/67779323-c331b880-fa64-11e9-9a2f-97730a89a6d6.png)

Commits
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5f19501 [TwigBundle][exception] Added missing css variable to highlight line in trace

src/Symfony/Bundle/TwigBundle/Resources/views/exception.css.twig

@@ -28,6 +28,7 @@
     --shadow: 0px 0px 1px rgba(128, 128, 128, .2);
     --border: 1px solid #e0e0e0;
     --background-error: var(--color-error);
+    --trace-selected-background: #F7E5A1;
     --highlight-comment: #969896;
     --highlight-default: #222222;
     --highlight-keyword: #a71d5d;