Site decoration is not a ranking factor. Sorry. It just isn't.
I have said it many times, the term rank has been confused over a period of time where rank is no longer seen asa very specific thing which it is. Rank is limited to certain factors that can be measured outside of the SERPs. Can improving a sites appearance and usability improve a site performance? Yes! Of course it can. But does any of that tie to rank? No. Sorry.
Search engines are comprised of two major parts, the index engine and the query engine. While search engines are are more complex than that, with just two major parts, it is important to know where rank lives. Rank lives in the index engine. SERP performance of any page depends upon specific queries and lives in the query engine. The only bridge between the two are the queries made by the query engine to the index.
Assuming that a sites content, hierarchy,or linking structure does not change as the OP has described, then any change assuming within the realm of reasonable, will make little to no difference. Why? Because the remainder lives within the realm of user engagement.
With the exceptions of bounce back and time on page, much of engagement cannot be measured dynamically. Remember that bounce back and time in page are SERP metrics. User engagement as it exists are largely subjective signals from human testing. For example, common engagement signals are whether there is an image at the top of the content, the content is above the fold, there is a login form,there is a subscribe form, etc. Much of these things can be seen by the render engine on the index side, however, these are signals and not ranking factors. No page can actually be measured for actual engagement. The search engine cannot know if people are signing up or clicking a link for example.
What ranks a site and pages of a site are provable metrics gathered over time. For example, a sites trust score, inbound links, semantic scoring to include expertise, grammar, reading level, the sites hierarchy, internal linking signals, authority, etc. SERP performance remains within the SERP metrics. Ranking factors remain within the index. Engagement is truly neither in that it largely effects neither. Not directly.
Can a sites decoration improve performance? Yes. Does it effect rank? No. However, it can indirectly effect metrics that do have an influence in the SERPs. But please keep in mind that these are relatively minor bumps in position as any comparison is weighed against other similarly performing pages.
Is it worth making your site better? Oh heck yes! The longer you can keep a user on site the better. Top of mind presence will begin to take hold and over time with enough satisfied users, a sites brand can be established. Otherwise, simple sites can rank extremely well. One of the best examples of this is The Drudge Report proving my point. This site is comparatively ugly and yet is one of the hottest sites on the web and has been for a very long time proving that a sites decoration has nothing to do with rank but has everything to do with other factors.