Hi @tur55. As others have already indicated, it is not possible to do this in Arduino IDE 1.x.
Arduino IDE 1.x does have a theming system, which allows you to configure the colors and fonts. However, the IDE doesn't have any knowledge of which variables are local vs. global, so there aren't separate theme settings for each variable type. In addition, the font is only configurable for each panel of the IDE window. At the sub-panel granularity, only the color is configurable.
Documentation of the Arduino IDE 1.x theme system is available here:
https://github.com/per1234/themetest#theme-documentation
This screenshot demonstrates the available settings:
Even though the capability you hoped for isn't available, maybe you will find it useful to clearly differentiate some of the other elements that are configurable via a custom theme.
Arduino IDE 2.x has a completely different theme system. Although its theming system is generally more capable and intelligent than the Arduino IDE 1.x theme system, I still don't think it provides the capability to style local vs. global variables differently.
You can learn about the Arduino IDE 2.x theme system here: