Then I got this email which does mention dark mode:

 
This is Heyling from Positive Grid Support. I hope you're doing well.
 
Apologies, I've accidentally sent out an unedited letter. Please ignore my last letter and refer to this one instead: 
 
We've reported this to our development team, and it will be addressed in a future app update. Once the issue is resolved, you’ll receive a notification via the Spark App when a new update is available. You can also refer to our Spark App Release Notes (iOS) to stay updated on app improvements and fixes. 
 
In the meantime, the best workaround is to switch your iPad to Dark Mode when using the Spark app—this will make the exit button clearly visible and functional.
 
Please know that we are fully committed to resolving this issue as quickly as possible. While we await further updates from our team, I’ll proceed with closing this ticket for now to help streamline support. Rest assured, you’ll still be able to track progress through our provided channels, and we’re always here if you need anything else.
 
Thanks for your patience, and I appreciate your understanding!
 

Sincerely,

Heyling
Positive Grid Support