Remove brand color from theme.

We only use it for the login button and the logo. We could also just hard code the color in the logo SVG and use one of our default button variants for the login form. Currently the login button is a special case and we just overwrite the button colors with the brand color. A side effect is that the login button color does not change on hover.

Start by locating the theme definition, login button usage, logo SVG, and default button variants in the TypeScript design system. Confirm how the brand color is applied and how hover styles are defined. Done means the brand color is no longer a theme special case, the logo retains its color, and the login button uses a default variant with hover behavior.

Written by the indexing model from the issue text.