How can I change the page title font and weight

Hi,
I’m loving the Singularity theme, but the page title font is too large and weighted (bold) for my needs. I can turn it off on the homepage template, but I’d like to use it. How can I change it? I’d prefer to write a custom css rather than editing the original css, if that will work. I looked at the theme css, theme parts/headers, and I can’t figure out the proper site code for the first line. (Too much of a novice here.) Can you help?

I tried .site and .Singularity with various default and title combinations, but they didn’t work. Thanks for any help!

What goes on this line?
h1 {
color: #000000;
font-family: nunito;
font-weight: lighter;
font-size: 38px;
line-height: 1.5;
}