Condense WordPress Theme Layout

My current WordPress theme wastes a lot of vertical space when a page first loads, and the default scroll bar behaviour feels clunky. I need the template code adjusted so that content sits higher on the screen and more information is visible above the fold without compromising responsiveness or breaking any existing plugins.

Here is what I’m after:
• Trim or refactor the CSS/PHP that adds excess padding, blank rows or spacer blocks, then tighten up each section so the page feels compact yet readable.
• Ensure that the new layout flows smoothly on desktop, tablet and mobile, keeping the header, footer and sidebars intact.
• Improve the scroll experience—whether by tweaking CSS overflow rules or suggesting a light-weight script—so navigating long pages feels natural.
• Deliver the changes as a child-theme or clearly commented code snippets so future theme updates won’t overwrite the work.

If you are comfortable diving straight into theme files (functions.php, templates, style.css) and you’re confident you can show tangible space savings without affecting site stability, I’d love to see a brief outline of how you would approach it and an estimated turnaround time.