A fair worry: apps have a reputation for bloating Shopify themes. Fundcap is built to be the opposite.
It renders from data already on the page
Your campaign's numbers (goal, progress, deadline) are stored as product metafields, which are part of the page Shopify already sends. The widget reads those directly, so it does not make a network call to draw the progress bar. The only script is a tiny one that animates the bar and ticks the countdown, and it fails silently if anything is off, so it can never break your product page.
It is an app block, not a theme edit
Fundcap does not rewrite your theme files. It uses Shopify's app-block system, which means the widget only loads on pages where you placed it, and uninstalling removes it cleanly. There is no leftover code and no global script running on every page of your store.
SEO is unaffected
The widget is standard HTML rendered server-side by your theme, so search engines see your product content normally. Nothing about a campaign hides your product from Google or changes its URL.