Customize auth page

### Issue type
Styling css of login page

**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:**
I was able to remove arrow icon from left top corner by adding below in css file
`/deep/ .nb-theme-cosmic nb-card-header { display: none !important; }
`

**Expected behavior:**
I want to remove the nb-card thing which is in login.template.html which is part of node modules so that it looks like below:

**Steps to reproduce:**

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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