Hi Raissuli!
The “scroll to top” button is an image, thus it can be changed as an image only – you can upload a new one in the “Scroll to top button” field in the bottom of the “Page Elements” tab of the Theme Options panel. I know, it’s tedious, but it’d be very difficult to organize the button customization in another way; although, when you’ve brought up this issue, I may have an idea how to make it react to colours, yet it will take some time; in the meanwhile, if you need a button with particular colour and transparency, you can tell me and I’ll make a custom blueprint image for you.
Best,
Andrew | Satori