Streamlining theme

HI, I have a theme which I’ve been customising. I’ve heard alot about ‘streamlining your theme’- I guess this means removing any of the unnecessary ‘options’ that the theme came with (once you’ve worked out which ones you want to use).

But how exactly is this done? Are there programmes that I can run for this?

Also- any similar information on how to “minimize your HTML and CSS and JS” would be greatly appreciated!

Thanks!