feat(style): adjust button styling, paddings, add css variable

[silvester-pari]

Implemented changes

This PR introduces

• fixes for button styling, alignment, padding
• introduction of a css variable --navigation-button-display, if set to nonethe left/right buttons are hidden

Screenshots/Videos

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[santilland]

I would propose to have the play button not live on the same line as the label and previous / next buttons, and personally i liked the centered look.
You have in any case the larger picture of how this integrates with styles and other things, so if having them left aligned fits that better it is ok for me :)