Use the same opacity for the tour backdrop as it's used in other places in XWiki

The opacity of the .tour-backdrop is 0.8 over #000 color. We should make it 0.5 in order to be consistent with the .xdialog-screen. See attached images

Prerequisites

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Try to reproduce the issue described by the task. You should use your browser's Developer Tools to identify and test your solution.

Once you have reproduced the issue presented in the task, it's the time to identify the location where it needs to be fixed in the code.
The sources are available at https://github.com/xwiki/xwiki-platform . You should be familiar with using Git locally or you might be able to commit your changes from the GitHub interface.

The task will give you hints about the component where the code with the problem is located. Identify the file, make the changes and submit a Pull Request for mentors to approve and close the task. Don't forget to add screenshots with before and after your changes.

Objective

(re)Use the same value for the Tour backdrop opacity.

Requirements

• Knowledge of CSS
• Knowledge of HTML
• Knowledge of XML
• Knowledge of Git
• Installing custom extensions on top of XWiki
• You will encounter concepts like Data Model, Skins Extensions etc. but you don't need to be familiar with them in advance

Expected Outcome

• Pull Request on the https://github.com/xwiki-contrib/application-tour repository with the solution
• The summary of the Pull Request must have a first line matching the JIRA issue following by a colon and then the title of this issue
• Following best practices defined at http://dev.xwiki.org/xwiki/bin/view/Main/WebHome