matt on 29 May, 2018 05:02 PM
That's not officially supported at the moment. It's something I'd like to add, but in the context of allowing different font sizes, text box dimensions, etc, and not simply setting the marker color to transparent. Markers don't have a transparency attribute at the moment so it it would be more complicated than simply adding a transparency slider to the color picker.
I can't guarantee this will work long-term, but you can set the marker icon to https://caltopo.com/static/images/blank.gif using the URL field at the bottom to make it transparent. Unfortunately the blank.gif image is only 1 pixel large, so it's pretty much impossible to click on in the style picker, in order to later set the marker to something else. Because the style on newly created markers is retained for the next marker you create, I'd recommend setting / unsetting the style to blank.gif using the bulk options feature, rather than the marker dialog.