I haven’t figured out a solution for the editor, but here’s one for the preview:
div.markdown-preview-view li.task-list-item {
text-indent: -27px;
margin-left: 0;
}
Put that in obsidian.css just inside your vault, and make sure custom CSS is enabled under the appearance settings. If you decide to use another theme, you’ll need to paste the above into it, and maybe play with the -27px to fine tune things.