Supermium

[Dave-H]

Glad you like Supermium, I've been using it for ages now on XP, and it's become my standard browser even on Windows 10.

To answer your questions -

1. Unfortunately, Supermium has no internal mechanism to set itself as the default browser on XP.
    It can be done with a lot of tedious registry editing, or by using a utility like PortableRegistrator.
https://www.majorgeeks.com/files/details/portable_registrator.html

2. This one is easy!
    Just type chrome://flags/#custom-ntp into the address bar, enable it, and fill in the URL that you want your new tabs to display.
    Restart the browser and there you are!

3. All I can suggest for that is to try different themes. Hopefully one of them will make things more readable for you.
    I'm sure that almost everything can probably be customised if you know how, but I don't have that level of knowledge of Chromium customisation I'm afraid.

Cheers, Dave.