Search Button from header changes its background color on ngx-admin demo

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**
The search button (on the header) of ngx-admin changes its background color when you change pages. You can see this bug on the ngx-admin demo too.

**Steps to reproduce:**
Navigate trough sidebar menu and watch the search button from change its background color. It is like a hover effect

Here is a link with the search button bug:
[Image to bug](https://imgur.com/a/sZAP8Un)