Child of SM Market theme for Magento 2

Hi, we installed the latest version of the SM Market theme at a Magento 2 installation.
We also created a child of this theme and this all seem to be oke.
Question: what is the best way to extend and/or overwrite files of this theme?
F.i. what is the best way to add extra css and/or JavaScript?
Where can we find more info/documentation about this?

Thanks for any help.

Hi,

Thanks for your quick reply, I know that document.
But unfortunately it looks like it’s not possible with the SM Market theme to simply extend css by a file like: app\design\frontend\Deonet\smmarket_child\web\css\source_extend.less

Remark: Deonet\smmarket_child is our child of the SM Market theme.

F.i. if I have only this in _extend.less: h1 {color:red;}, then the h1 tags on the frontend still are dark grey (I guess the default SM Market color for the h1 tag).

If I have this in _extend.less: h1 {color:red !important;}, then the h1 tags on the frontend are red.

But of course we don’t want “!important” in every tag.

Do you have more ideas, help, info?