Why changing theme settings does not take effect?

I changed Logo file and Footer text on the page “admin/themes/config?name=default”, but in a incognito window, I can still see the old logo and footer text on the login page.
Edit: Later I found the change took effects on frontend pages, but not all pages including the administration pages and login&register pages, which are also what I want.

Themes, and therefore theme settings, only affect the frontend pages.

So, could you please tell how to modify administration theme, including login&register pages?