IFRAMEs are designed to ensure CSS isolation and code integrity, which means you have no direct control from Webflow outside of the embed code snippet.
Sometimes the IFRAME itself is in that snippet, and you can change some aspects- sizing, scrolling behavior, border. Other times, the IFRAME is created by a script which means you're even more limited in what you can access.
To style content within an IFRAME, you generally have to style it at the source. If you own the website you're IFRAMEing, you can do that.
If you're IFRAMEing a 3rd party widget such as a scheduling system, then you're limited to whatever styling capabilities they give you.
Most services offer some limited styling on color & font choices, and button styles.
Some services give you the ability to customize CSS;
If you want precision control over how your embedded content appears, and know some CSS, this is what you need.
And as a Webflow dev, this is almost always a resounding YES.
