[ZEPPELIN-4428] Fix bug - Highlighted line number disappears

[milooy]

What is this PR for?

Fix bug that highlighted line number disappeared.

(Before)

(Problem)
In paragraph.css file, there was a custom css that changes background color of the line number.
Need to apply that css line to this UI together.

(After)

What type of PR is it?

Bug Fix

Todos

• - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-4428

• Open an issue on Jira https://issues.apache.org/jira/browse/ZEPPELIN/
• Put link here, and add [ZEPPELIN-Jira number] in PR title, eg. [ZEPPELIN-533]

How should this be tested?

• First time? Setup Travis CI as described on https://zeppelin.apache.org/contribution/contributions.html#continuous-integration
• Strongly recommended: add automated unit tests for any new or changed behavior
• Outline any manual steps to test the PR here.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update?
• Is there breaking changes for older versions?
• Does this needs documentation?

[zjffdu]

Thanks @milooy for the fixing, LGTM

[Leemoonsoo]

LGTM