you guys

People use "you guys" when talking to a group of people that may include men and women. For example, a waiter might say this to a large family eating at a restaurant:

Have you guys been here before?

English has a problem because "you" is used for both one person and a group of people. It can be confusing. To fix this, people in different regions use different words to talk to a group. In Southern areas of the U.S., some people say "y'all":

Have y'all covered exponents yet?

People in other areas of the country think this sounds really dumb, though.

"You guys" is still considered casual English, but it's very common for younger people and people from the West Coast of the U.S. to use this phrase.

Meanwhile

You use the word "meanwhile" to describe something that's happening (or will happen) at the same time as something else. This word is often used when telling stories. For example, in this scene from The Last Dickens: a Novel, two actions are going on at the same time:

As Mason made his way around the thick shrubbery he jumped out at Turner's signal and displayed his sword at the surprised Narain, who put up two trembling hands and lay flat on the ground. Meanwhile the other thief had pushed Turner down and dashed into the dense trees.

In the example at top, the speaker wants the group members to make a decision at the same time that he is at the store shopping for supplies.

decide on (something)

To "decide on" something means to make a final choice about something. For example:

It took several months for us just to decide on a new name.

theme

A "theme" is an idea that unites all the parts of a project, a book, a film, or a party.