The current version of the site editor enables you to add multiple sections to a site, along with several other improvements.
In order to maintain precise backwards compatibility by default, existing themes will include some compatibility CSS that affects text sizing. Whether or not a theme has this CSS is controlled by the setting "Text Backwards Compatibility"
From the Theme Editor, on the search box type "Text Backward Compatibility"
The advantage to having this setting set to "True" is that there will be no style changes due to the new update. This is the default for themes created prior to the release of current of the site editor.
The advantage to having this setting set to "False" is as follows:
Text sizes will default to the same size for grid sections regardless of their number of columns. (If left as True, text sizes will be larger in 3 column grids and smaller in 5 column grids)
Text sizes in text sections will default to the same size as in the grid section.
Text sections will have padding and margins that look better when included as a section in a subpage.
Text sections will default to using transparent backgrounds (instead of always being white)
Grid sections can be any exact width (instead of snapping to the nearest 30th pixel)
If you are working with an existing site and want the above advantages, you should consider switching the site's theme to a theme which has "Text Backwards Compatibility" set to "False".
If you are considering updating an existing theme to set "Text Backwards Compatibility" to "False", you should make sure that any sites which use that theme are still looking good after the update. Alternatively, you could copy the theme and switch the setting to "False" for the new copy of that theme.