Theme Question 2.5.1

DkS

Ok i have read over the docs for veserion 2.5 for a few days I have started working on themeing for my site and I want to use the css grid I runn on my site. my question is how do I let the theme do the mobile instead of selecting a mobile theme?

I already have my css and js

this is my addon theme

{
"name": "mytheme",
"description": "The default theme for mytheme.",
"version": "2.4.201",
"hidden": false,
"key": "mytheme",
"type": "theme",
"priority": 1000,
"authors": [
{
"name": "Josh",
"email": "me@me.com",
"homepage": "https://me.com"
}
],
"layout": {
"categories": "Modern",
"discussions": "Modern"
},
}