Add forum styles into basic.blade.php

[mbakirov]

Changes proposed in this pull request:
Add forum styles into confirm email page would solve a bug that appears when the dark theme is turned on and theme was slightly customized.

For instance:

• dark theme is on
• primary color is set to #ffffff
• secondary color is set to #151515

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)