add CTFd themes

[noraj]

Some themes that can be added in the repo:

• https://github.com/noraj/ctfd-theme-sigsegv2
• https://github.com/stormctf/CTFd-Theme-StormCTF
• https://github.com/tamuctf/ctfd-theme

[Anon-Exploiter]

+1

[Z3RO-O]

https://github.com/hmrserver/CTFd-theme-pixo
I have seen most people using this.
Best to add it in official Themes.