Moodle's default theme is a very neutral grey/white theme.
This theme is always the most up-to date.
Programmers working on Moodle should always design for this theme, and add new styles to this theme.
By default, these styles are automatically loaded before any other themes, so that all custom themes need to do is to define their own styles to augment or overwrite particular standard styles.
In this way custom themes are not "left behind" by Moodle upgrades.