Hey there, you can remove the dotted line with this CSS
.markdown-preview-view li.is-collapsed,
.markdown-preview-view .markdown-preview-section > div.is-collapsed > * {
background-image: none;
}
And you can change the size of the collapse indicator by changing these values:
.markdown-preview-view .collapse-indicator > svg.right-triangle {
width: 14px;
height: 14px;
}