Fix for issue #7565 on Click to add a cell button not in background

[aj202088]

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[github-actions]

👈 Launch a Binder on branch aj202088/notebook/main

[rsaditya01]

Welcome @aj202088,
Thanks for looking into the issue and contributing to Jupyter Notebook.

On pulling your changes in local and testing, the updated notebook UI looks different from the desired output.

(A) Notebook UI after change -

(B)Desired Notebook UI (mentioned in the issue)-

The notebook UI in (A), keeps the "Click to add cell" button in the footer area, the button appear all the time below the notebook area and the button spans to entire width of 'inner notebook area (white area)'.
While the desired output is that the "Click to add cell button" can be placed inside 'inner notebook area'(as mentioned in issue description #7565), width should be equivalent to Notebook Cell width and it should appear only "on hover" over that button (current functionality).
Could we look into making some changes in "inner notebook area" and placing the the button there with minor alteration in css.