Found that this css from this post - Default theme - changing Headers size as a snippet - changes the colour of H2 - H6, but not H1, for some reason, and doesnt change the size of any of them.
- << EDITOR and PREVIEW HEADERS >> */
.cm-header-1, .markdown-preview-view h1 {
font-size: 1.55em;
color: #797972;
font-weight: 400 !important;
}
.cm-header-2, .markdown-preview-view h2 {
font-size: 1.4em;
color: #797972;
font-weight: 400 !important;
}
.cm-header-3, .markdown-preview-view h3 {
font-size: 1.3em;
color: #797972;
font-weight: 400 !important;
}
.cm-header-4, .markdown-preview-view h4 {
font-size: 1.2em;
color: #797972;
font-weight: 400 !important;
}
.cm-header-5, .markdown-preview-view h5 {
font-size: 1.1em;
color: #797972;
font-weight: 400 !important;
}
.cm-header-6, .markdown-preview-view h6 {
font-size: 1em;
color: #797972;
font-weight: 400 !important;
}