Unify buttons' styles

[tiiavalt]

Is your feature request related to a problem? Please describe. There are currently many button elements and their styles are repeated around the code base.

Describe the solution you'd like Let's unify the buttons' styles and possibly create a reusable button component.

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The result: Created two new styles for buttons: DefaultButton and ToggleButton in button.styles.js file. There are no UI pictures yet for the toggle button, so current styles are only a guess what the button should look like, but at least now the styles are more consistent with the rest of the UI.