Thunderbird 60.6.1 . TB came with default themes a dark and alight. I have downloaded a compatible dark theme I like much better than the default theme. In years (release versions) past we had a remove button in the ad-Ons app to remove themes wee had downloaded and installed. The current TB version only has a remove button for the themes I have installed, the default themes have no method of installing. I have tried all the suggestions available on the internet to no avail. Most of the suggestions do not address TB 60. I cannot even find where the themes are installed in Windows 7 to rename. Yes, I can open Ad-ons > Themes and change the theme anytime I want. I use Calendar for job appointment dates and addresses, when the date arrives I print out the calendar page for use with GPS locations. Printing an entire page in black uses a lot of black ink, so I have to switch to a lighter theme before printing. With four themes in Ad-ons it becomes less usable as it is, with only two themes it becomes a lot easier to switch back and forth. The best approach would be to have a button on the calendar' day page to switch from colored theme to B&W theme, but that might force the printer ink manufacturers into bankruptcy. Sorry if this is the wrong place to post about Thunderbird, but this is where your online support pointed for contact.