I found the solution on another WordPress support post. There seem to be many people with this problem using many different themes, not just twenty-fifteen. I tried about 5 of the .css file editing solutions suggested in various posts, but only this one worked:
Note: I am currently using Twenty-fifteen version 1.2.
1) Find this text in your wp-content > themes > [go into your theme's folder] > style.css file under the section called 4.0 Elements:
ul,
ol {
margin: 0 0 1.6em 1.3333em;
}
2) replace the above code with this
ul, ol {
margin: 0 0 10px;
padding: 0 0 0 15px;
}
Save the edited .css file, and then go to a page of your blog that has the problematic bullets on it, and refresh the page to see if it worked.
Of course, the experts here will always caution you that changing the main theme style.css is not the best solution as the .css file will revert back whenever you update your theme. Better to use a “child theme” and edit its .css file. However, if you don’t yet have a child theme set up (my case at present), and this fix works in the main theme .css file, you’ll at least know that it works before you go to the trouble to set up a child theme with this edit.