The error has crept in by extending/enlarging the input fields when using var(), clamp(), etc. and will of course be fixed in the next version.
However, you may have to re-enter the values, as the excess brackets are written to the database when saving. Therefore I would refrain from setting clamps or vars - generally anything with brackets - in RC2. This applies to all input fields, not only in the theme styles.
The issue has been fixed with Bricks 1.4.
The only problem that may occur now is that the broken variables (or functions) have been stored in the database and therefore do not work anymore.
So if you run into a broken layout somewhere, it is most likely due to a broken value in an input field like “var(var(–variable)”. Sorry for the trouble.