How to get rid of unused css & java script without any plugin?

Hey there guys,
I am kinda new to this sort of thing, and I have been wanted to get rid of this unused css and unused javascript thing for quite some time now. I have hired one or two developers in the past, but the problem remains.
Could you guys help with this problem. It is decreasing my site’s speed.
My website revolve around succulents so I used a lot of graphics. It is already taking a toll on my website, and the last thing I thing I want is this unused css thing.

Help me guys. THANKS.