CSS no longer formats h1

Hello,

The CSS below is what I got from your team a few years ago. After updating Sydney, this code no longer formats h1 (both on pages and on posts) while still formatting h2 and h3 as they did with the previous version of the theme.
Could you make a necessary change or give me an entirely new code?

Thank you in advance,

h1:not(.site-title) {
      padding-bottom: .5em;
  border-bottom: 2px solid #9fdf9f;
}
h2:not(.site-description):not(.maintitle) {
  padding: .25em 0 .5em .75em;
  border-left: 6px solid #9fdf9f;
  border-bottom: 1px solid #9fdf9f;
}

h3:not(.widget-title){
  position: relative;
  padding: 0 .5em .5em 1.7em;
  border-bottom: 1px solid #9fdf9f;
}
h3:not(.widget-title)::after {
  position: absolute;
  top: .4em;
  left: .4em;
  z-index: 2;
  content: '';
  width: 12px;
  height: 12px;
  background-color: #9fdf9f;
  -webkit-transform: rotate(45deg);
  transform: rotate(45deg);
}

The page I need help with: [log in to see the link]