It’s a good time to learn, if you want to make customizations. 😉
See in the right column that screenshot; it tells you exactly what in the theme is effecting that area. This is what you’d do in your inspector by doing Right Click > Inspect Element either in your Firefox or Chrome web browser.
So, the line used here is:
.header_logo .tagline { ... }
And so in order for you to override, you need to either match it or write something more specific. So this:
.tagline { ... }
… would be less specific. However, this would work:
.header_logo .tagline {
font-style: normal;
}