Hi folks, I noticed this today when I was trying to show off the new TS6 beta to a friend in-person.
The UI for “Continue to external site” is not visually clear.
When clicking on an external link, we both could not find the accept/cancel buttons for a good 2 minutes. These buttons blended in quite well with the background content.
I have a UI mockup that demonstrates what I am talking about.
The below screenshots were taken today on TS 6.0.0-beta4 , build # 1777360703
Screenshot 1 - What we have today:
Screenshot 2 - My UI mockup:
As an addendum - light mode is not as bad, but it would greatly benefit from a UI tweak and keep the experience consistent across current default themes.
EDIT: i went back into the client after posting, as I wanted to see what this looked like in high-contrast mode, but alas I can’t find that 
so assuming none exist at the moment.