The reason that our flowchart is all gray is that all of the boxes are Style12. When we changed themes, they changed from the Littoral (default) theme's Style12 to the Shrubland theme's Style12. Let's add some of these Quick Styles to our flowchart.
While holding the Shift key, click on the start and end processes (the ovals) to select them both. Then, click the Quick Styles button and choose Style7. Using the same procedure, make the processes (the rectangles) Style10 and the decision (diamond) Style9.
That looks much better! But let's say you decide this isn't the right theme. Try Benthic instead. Notice that the Quick Styles you applied are not lost; instead, they are changed to a parallel Quick Style that corresponds with the new theme. However, this is not true if you simply applied a fill, line style, or effect using those controls on the Home tab. Everything but Quick Styles is overwritten by a new theme.
Now all that's left to do is to share your masterpiece.