You can look at the value set in the options dialog box as if its an (invisible) assignment (a normal one, not a global) right at the top of your worksheet. As with any other variable, this definition can be overwritten in the midst of the sheet but this does not change the definition at the top and so does not change the value in the options menu.
You see 10^-4 in the menu and this is the valid value of TOL in your sheet up to the line where you define the new value 10^-5. You can't change the intitial default value in the options menu from within the sheet. But if you want to be sure that a specific value is valid from the very beginning and don't want to bother with the menu, you could either place a normal assignment to the very top of the sheet yourself or use a global assignment anywhere els in your sheet. You can override a global assignment anywhere in your sheet by a normal or another global assignment. If you use multiple global assignments then the last one is the one which is valid at the top of the sheet.
In other words, no, no special font necessary to change system variables. They behave exactly the same as normal variables. The only difference ist that they are invisibly defined at the very top.
WE
