Thank you for your post.
A blue border is displayed when an object is selected. What Theme are you using? What objects are you using?
Another user has also requested the blue border be darker, so I have attached your post to that customer's request. I also recommend that you enter this suggestion into our Feature Requests web form at:
Entries into this web form are read by Development and Product Management where the entries are then discussed and considered for a future release. Although I could copy your post and paste it into the web form, there are a couple of questions asked that only you can answer.
A darker line won't resolve this issue. The layout still might be the same or nearly the same as that "darker" color. FileMaker needs to either add code that computes a proper contrast to the border color or they need to go back to the standard selection handles.
I'd prefer reverting to the original selection handles on every selected object myself.
>> I'd prefer reverting to the original selection handles on every selected object myself.