How can I overcome design limitations with the Envy theme?

Store Design
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I’m building a site using the Envy Theme and have run into a few small design caveats that I would like to have control over:

  1. Changing the content width to not be full-width
  2. Changing the type size of the subheadings
  3. Adding a dark overlay to the background image of the “Image with Text Overlay” sections
  4. Adding a dark overlay to the background videos of the “Video with Text Overlay” sections
  5. Adding a logo to the footer

This is my first time building a site with Shopify so I’m not used to having these limitations, and I know the limitations vary per theme so it’s hard to find solutions regarding Envy. Is there CSS to make these changes, or recommended apps? Any help regarding any of these would be appreciated! Thank you.