fix: guest navbar button theme colour

src/webapp/components/navigation/guestNavbar/GuestNavbar.tsx

···
               <Button
                 component={Link}
                 href="/login"
-                color="dark"
+                color="var(--mantine-color-dark-filled)"
                 rightSection={<BiRightArrowAlt size={22} />}
               >
                 Log in