Appearance Settings - Colors
The Colors settings control your timeline's background theme, the colors its events use, and where each layer's color is applied.
To reach these settings, click the Appearance button above your timeline and open the Colors section.
Background
Timelines are white by default, and you can switch to any of the other themes here. A theme sets the background color, the font color, and the timeline lines together.
For example, here's the default white theme:

And here's the same timeline with a light gray background:

Custom Event Color Palette
Enter a comma-separated list of HTML hex colors (like #1565C0) and Preceden applies them to both new and existing events.
Layer Colors
These two settings control where each layer's own color is used:
Apply the layer color to the layer's background
With this on, a layer's color fills the layer's background as well as its name.
For example, here's what it looks like with the layer names at the top:

And here's the same timeline with layer names to the left:

Override event color with the layer color
Turn this on to make every event in a layer use that layer's color instead of its own.
For example, here's a timeline with layer names to the left, no layer background color, but overridden event colors:

And here it is again but with the layer color applied to the layer background:

Misc
Display past events in gray
On a project plan, graying out past events makes it clear at a glance which ones are done. Rather than recoloring each event by hand as it finishes, turn this on and Preceden grays them out for you.
Darken weekends
This option darkens weekends on your timeline to make them stand out. It has no effect at large scales (like years or decades) or very small ones (like minutes).