How do themes differ from periods as tools for understanding history?

The difference is how the time can affect the concept of both.

In History a theme is a repeating moment, a common idea that keep repeating itself in the past. This is called theme.

A period is a time when the themes were set in. The period represents a specific time were something occurred, with a specific beginning and ending.

The theme can be found in different times.

c it has to be i googled it

b because thats the naswer i got