Revise color configuration in conference config

Some colors in the conference configuration should not be necessarily configurable (e.g. `DARK_PRIMARY_TEXT_COLOR`, `DARK_SECONDARY_TEXT_COLOR`..., which are derived from the Material Design Guidelines).

Some text colors set to "white" (e.g. `LIGHT_TEXT_COLOR`, `ANDROID_ACTION_AREA_FOREGROUND_COLOR`, `WINDOWS_ACTION_AREA_FOREGROUND_COLOR`) should not be configurable, because the app assumes a dark theme for these cases and white provides best contrast.