Add helper to check if a high-contrast color scheme is in use

If a high-contrast color scheme is in use then we want to avoid the
usage of custom colors. Since we allow overriding the system's
high-contrast mode setting by configuring a custom color scheme we need
to check if the default scheme is in use in addition to checking the
status of the high-contrast mode.

• GnuPG-bug-id: T7610