Change font and font size of titles

Hello,

I have been busy all afternoon trying to find out how to change the font and font size of the post and page titles. The body text is in arial font, but the titles are in a sans-serif font ( i think times new roman?). I would like to change the titles also in arial font. The titles have also a very big font size. Where can I reduce the size.

I have been scrolling in the support pages, but I could find an answer that worked out for me. I work with custom CSS and already tried several things, but than only the size and font of the body text changes, but not the titles. Can somebody please help me with this issue. Thanks a lot.