Tweak existing WordPress site

I have an existing bilingual WP site.

It is a business directory site running the eList theme that has been tweaked quite a bit.
It runs the Polylang plugin for Hebrew and English.

All work can be done on the English site, and then implemented (by you or someone else) on the Hebrew site.

I want to:
1. Tweak existing functionality.
2. Install and tweak 2/3 plugins that I have already installed (MailPoet, advertising banner plugins I need to choose from, AdRotate)
3. If you know CSS/HTML this is an added plus as I also want to tweak the current UI - CSS/HTML stuff.