EmailPasswordSignInPage should use theme primary color for Sign In button.

Currently the EmailPasswordSignInPage uses a FormSubmitButton for the Sign In button UI. However, this does not accept a custom color or a context (in order to use the theme primary color). As a result this widget is not usable when the color indigo does not fit the rest of the UI being presented.

It seems like it would be better to allow a context argument so that the UI can use the parent theme.

Locate EmailPasswordSignInPage and inspect how it uses FormSubmitButton for the Sign In button. Check the button and its parent theme handling first; done means the page can use the theme primary color through a context argument instead of requiring indigo.

Written by the indexing model from the issue text.