"A dark room in a bright house"
Translation:Ciemny pokój w jasnym domu
W (in) requires the locative case. Jasny (like other masculine nouns) declines like this:
A. jasny / jasnego (animate)
Look at the declension for other genders here (but make sure to choose Polish and not Lower Sorbian, I spent a minute wondering at those forms before I realised :))