For the Default layout, many of the cells look very similar, and so they use a common Global Property set. This makes it easy to change the font or font size or text color, etc, in one place and have it take effect in many places. Your most recent screen shot confirms that when you edit the "Round" cell, it is using the "Even Rounds" global property set. But a lot of cells, as you have seen, are also using that global property set. If you change it, the change will affect all cells using it.
Instead, when you edit the Round cell, uncheck "Use global property set". Then set the font/size/colors you want.