test_events: Rename functions to use dark_theme and light_theme.
test_events: Rename functions to use dark_theme and light_theme.

This commit renames the tests for changing logo in test_events
to use dark_theme and light_theme instead of night_mode and
day_mode.

andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip