I’m always astonished by companies that say that culture is important, but do no work on their values.
Look after your values, and your culture will follow.
A strong set of organisational values, lived by the leadership and reflected in how your company does business will end up creating the foundations of your culture.
As people join and bring their own wonderful diversity, your culture will adapt for sure. Welcome this, but keep your values constant. Then the changes will only make your culture stronger and those core values deeper.
Start with values. Live and lead with them, turn them explicitly into how you do business, and your culture will look after itself.