I changed some colors in the palette (Background and theme --> Set color palette), but it has no noticable effect. If I add a new shape or line, it offers only colors from the old palette, and not the new ones I selected. What is this palette supposed to do? How do I change the default color offered for shapes?
Setting your colors will provide you with theme colors for your presentation.
After that, you will be able to choose from these colors and their derivatives.
If you wish to not just have the pallet available but to change the default colors of the elements, such as shapes or lines and arrows, you can update the style preset.