bug #13489 [VarDumper] CSS fix (Arrilot)

This PR was merged into the 2.6 branch.

Discussion
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[VarDumper] CSS fix

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Small css issue:
When you use `dump()` after some html has been already echoed (this can be common in non-Symfony app) and you have elements with e.g. `position: absolute` you can end up watching logos and menus instead of vardumper's output:
![css-issue](https://cloud.githubusercontent.com/assets/2826480/5841607/438bf874-a1af-11e4-9905-542a59d358f8.png)

`position: relative` and `z-index` help a lot and i don't see any disadvantages in having these styles by default

Commits
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538195c [Var-Dumper] css-fix

Author: fabpot

Dumper/HtmlDumper.php

@@ -31,7 +31,7 @@ class HtmlDumper extends CliDumper
     protected $headerIsDumped = false;
     protected $lastDepth = -1;
     protected $styles = array(
-        'default' => 'background-color:#18171B; color:#FF8400; line-height:1.2em; font:12px Menlo, Monaco, Consolas, monospace; word-wrap: break-word; white-space: pre-wrap',
+        'default' => 'background-color:#18171B; color:#FF8400; line-height:1.2em; font:12px Menlo, Monaco, Consolas, monospace; word-wrap: break-word; white-space: pre-wrap; position:relative; z-index:100000',
         'num' => 'font-weight:bold; color:#1299DA',
         'const' => 'font-weight:bold',
         'str' => 'font-weight:bold; color:#56DB3A',