Static modifiers are modifiers used by the engine. They regulate a number of settings that are not opened to scripting (the ai difficulty settings, base maximum rivalries, ...).
Static modifiers are global: they apply to the whole game and cannot be changed dynamically.
Vanilla values are configured in the files contained in the common/static_modifiers folder, which contains more than 1000 variables affecting many areas of the game.
How To Change Vanilla Modifiers
Place a changed static modifier from any of the core files in "common\static_modifiers" in a new file that comes before asciibetically.