You might need !important since there are many elements with higher specificity (those “win” over your declaration otherwise).
You could try something like
/* make all headers the same size */
h1, h2, h3, h4, h5, h6 {
font-size: 1rem !important;
}
(You can also use fractional numbers like 1.2rem.)
If above doesn’t work, here is another way:
/* make all headers in preview the same size */
.markdown-preview-view h1,
.markdown-preview-view h2,
.markdown-preview-view h3,
.markdown-preview-view h4,
.markdown-preview-view h5,
.markdown-preview-view h6 {
font-size: 1em;
}
/* editor headers */
.cm-header {
font-size: 1em;
}
And don’t forget to enable your oogie-custom under Settings → Appearance → CSS snippets!
Note: This might only affect preview mode, and maybe have no effect on edit mode.
Here’s more to read on absolute and relative lengths in CSS (the relative lengths can be scaled & zoomed):
Because Obsidian can—for simplicity—be thought of as a browser. 