There are no strict rules or regulations on dress in Rwanda. Though the country has a fairly western attitude particularly in Kigali, you should dress decently following the code for what to wear in African countries.
In more rural areas it will not be unusual for a woman to wear trousers but it may be advisable to dress modestly during your tour in Rwanda.
Are you taking a safari in national Parks, here are guides to what to wear;
- What to Wear on a gorilla trek in Volcanoes National Park
- What to Wear on a wildlife safari in Akagera National Park
- What to Bring on a Hiking Safari
- Cultural Etiquette for Rwanda Visitors