Hey Sofia,
The code you’re most likely referring to (see Secure Note) is an internal stylesheet. Those styles are coming from the Theme Options. It will not affect SEO. The most probable reason why your SEO specialist told you Google doesn’t like it is because of the Pagespeed recommendation to eliminate render blocking CSS. You should not worry about that. The reason why there is an internal stylesheet is because, there is currently no feasible method to output the theme options to an external stylesheet.
Please don’t worry about that. There are thousands of X sites that rank well. That is because how the site was built is not a ranking factor. Your content alongside performance are the major ranking factors.
Let me give you an example. Here is a website that uses X https://wpspeedguru.com/. It ranks well for the keyword wp speed. You see it ranks well because of the domain name and the keywords used. That demonstrates that X won’t prevent Google from ranking your site. It depends on your target keywords and your actual content.

X has been built from the ground up using the latest, semantic HTML5 markup, allowing search engines to easily distinguish where your main content starts and ends and easily distinguish what it is about. Semantic markup also means clean markup. X has been architected to avoid extra structural code and presentational items in your main content, which can cause problems as your site is being indexed.
Thanks.