[Feature/Improvement] Add visual cue for the halfway point

**Purpose:** As a user, I want to be aware of the halfway point while my device's volume is lowered.

**More details:** Currently, there is only an audio cue at the halfway point of the work interval. There should be a visual cue as well so that the user is aware of the halfway point even when the device's volume is lowered. Some ideas:

- Invert the background and text colors for a second. I.e. the background will become white and the text will become green.
- Animate some sort of partially transparent overlay.
- Flash or animate the seconds on the screen.

**Acceptance Criteria:**
- Visual cue implemented to show the halfway mark of each work interval.