Blank URLs in message sent

When I send an email from Mailspring to a friend in England using the latest build of Thunderbird any URL links I include in my emails do not display for him, totally blank. When digging deeper he found this:

I was playing around with your invisible URL message(s). Turns out that the HTML contains lines like:

color:rgb(255, 255, 255)

which is white and renders the URL invisible. If I save the message and then edit the source to rgb(0, 0, 0) then the URL is in black font and so becomes visible.

Is anybody else seeing this problem? I wonder if it is my “Dark” theme here while he must be using a default white them there? I’ll check it by switching my theme back to white.

When I switch my theme from “Dark” to “Light” the emails that I send are now visible on Thunderbird.

Why is there a " (https://link.getmailspring.com/link/C05533A … link directly after the URL?