Issue #11048: Prevent localization issues with font-size

This patch attempts to resolve issue #11048 (closed).

Depending on your system's locale, numbers may be rendered with either a "." or "," for the decimal symbol. CSS does not like commas used as the radix however. When we adjust the font-size in the GUI, we use printf () to print the font size float - and this can result in a font size of 1,2rem rather than 1.2rem. As reported, this breaks the theme when the CSS parser encounters it.

To prevent the issue, I used integer division and mod operations to get values on both sides of the decimal, then created the string manually with a period always in the middle.

@brunolopesdsilv, I think you had this issue as well. If you do, and when you have time, could you test and see if this fixes it for you?