WIP: UI Tweaks for the timeline

This MR includes several tweaks for the timeline.

• Adds a darker background for the timeline. This helps separate the parts better.
• Makes the dividing lines (spacer) more subtle, so they don't stand out so much
• Change the colors for Audio clips to the new HIG colors (Green 1 and Green 3).
• Also invert the contrast in audio clips so they look better with a dark background
• Add a CSS class for Title Clips and a specific color for them (orange 1 from the HIG)
• Change the keyframe and line color to the new HIG colors (yellow 1)
• Increase the dark overlay for keyframes and make the line a bit more subtle
• Use symbolic icon for zoom