Improve UI of Thyme video player

[Splines]

There are many points where the design of the MaMpf video player (called Thyme player) could be improved, e.g. the default video bar could benefit from some face lifting and the side panels as well. As inspiration, the Firefox in-built video player is quite nice in my opinion:

Main pain points are:

• Buttons not styled according to Bootstrap buttons, that is only the icon itself is clickable, not an area around it. This also affects accessibility as screen readers won't recognize these <i> tags as buttons.
• Colors should follow our branding
• scrollbars in the side panels are shown even when there is nothing to scroll
• the video timeline is not "smooth" enough, i.e. the button advances in too little discrete time steps. Same for the volume button
• Timeline could benefit from a gradient to make it more subtle