On the Themed Event screen, you will be shown the event missions, the overkill progression bar, and a leaderboard on the right side of the screen.
You can select which mission you would like to play by tapping on the mission icons and then tapping the PLAY button in the bottom right. Not all missions will be available to play immediately. You can unlock these by following the instruction at the bottom of the screen, or by spending Gold. Some missions will also allow you to earn more points than others. The max amount of points that you can get per attempt is shown just below each mission icon.
There is also an overkill progression bar in the top-left corner of the Event screen. This can be filled by earning points via missions. You can also lower the total by spending Gold. Once the bar is filled, you can activate overkill via the button just below the bar (if it is a clan event, an Elder or someone with a higher rank can only activate overkill). Once active, the max score for missions will be triple for an amount of time (applies to all players in the clan during clan events).
On the right, you will see an end timer, a cycle timer, and a leaderboard. The end timer indicates the remaining time for the event, once this time is up, the event will be over and no further contributions can be made. The cycle timer indicates when the missions will change. The ‘i’ button to the right provides further information on the current event. The leaderboard shows your position for the event along with the top three players/clans. Tapping on a player/clan will take you to the leaderboard screen providing more information on this, including rewards.
As there are multiple themes (with the recent introduction of Robot events) the theme will alternate every other week.