Is it impossible to change the colour of the login button for a member areas login pop-up form?

I can't see any obvious way to do this. This just seems bizarre to me - can it be changed with custom code? Ease of design consistency across a website is one of the reasons I may choose Squarespace for certain projects, so it's strange that I can change the button colours for my checkout form but not my member area login form.

Bizarre as it seems, that's correct. Without custom JavaScript you cannot style the popup windows for login and sign-up, the account panel (where members manage their account) or the access-denied page for non-members. It's one of the limitations that I shared with Squarespace in March in an effort to see some improvements.

If you're interested, you can view the list of Member Area limitations here. 

Thanks v much @paul2009 - fingers crossed this is resolved in a future release! 

Your list highlights some really important issues. It's simply not a viable product right now for my clients (who are largely in the UK and therefore need GDPR-compliance), which is a real shame as I would love to be able to keep everything in one platform.

I have tried editing this too, and custom CSS is just totally ignored...! Let's hope this becomes an editable thing in the future, a bit rubbish when you can apply branding colours to everything else on a Squarespace site!