How to change the CSS version

Hello

I have a vanilla site , i also do some css update to change the theme.

so i come acorss problem

my theme modify from the default theme

and when i change the custom.css , my user`s explorer just cache the custom.css

the one slovtion is change the about.php`s version ,and the js or css request will append the theme version number ,so the explorer will get the new css file .

but i use the default , and the verson number is vanilla version number

i try to modify the vanilla version in config.php ,and it not work , i change the about.php version in default theme , and it not work too.

css also append 2.0.18.4

so ~~~ how can i do it ? any help

By the way , is there any update for html static , static the content not change.