feat: add fill functionality for background color

[HrishabhCodes]

This PR is related to issue #4. I have removed the theme toggler and added the filling functionality to change the background color of the canvas.

fixes #4

Screen_Recording_2022-03-02_at_6.17.31_PM.mov

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[DhairyaBahl]

@HrishabhCodes You don't have to remove that toggle button ! You have to change the background color of canvas as a part of this issue.

[HrishabhCodes]

@HrishabhCodes You don't have to remove that toggle button ! You have to change the background color of canvas as a part of this issue.

Just to be sure with my tasks, I have to add the theme toggler again. Is the background color functionality added by me fine?
I have one more doubt regarding themes. What should be changed? Only the canvas or sidebar and header as well?