@looper: this is a way to change header sizes
/* Change header size in Edit mode */
.cm-header-1 {font-size: 28px; font-weight: bolder;}
.cm-header-2 {font-size: 25px; font-weight: bolder;}
.cm-header-3 {font-size: 22px; font-weight: bolder;}
.cm-header-4 {font-size: 20px; font-weight: bolder;}
.cm-header-5 {font-size: 18px; font-weight: bolder;}
.cm-header-6 {font-size: 16px; font-weight: bolder;}
/* Change header size in Preview mode */
.markdown-preview-view h1 {font-size: 28px; line-height: 118%; font-weight: 600;}
.markdown-preview-view h2 {font-size: 25px; font-weight: 600;}
.markdown-preview-view h3 {font-size: 22px; font-weight: 600;}
.markdown-preview-view h4 {font-size: 20px; font-weight: 600;}
.markdown-preview-view h5 {font-size: 18px; font-weight: 600;}
.markdown-preview-view h6 {font-size: 16px; font-weight: 600;}
If you are not interested in the font-weight you can delete those elements, but leave the closing } in.
Furthermore, if you click on the Gitlab link in the topmost comment on this page you’ll find many interesting snippets.