Search doesn't see that the site got prettier. It sees that familiar URLs and copy are gone.
That is why sites lose Google traffic after a redesign. The causes are the same every time: URLs changed without redirects, copy was cut to fit the layout, meta tags weren't carried over, and the block on indexing that sat on the test copy was never removed from the new site. Each of these can be closed off before launch.
Page URLs
Before any work begins, we list every URL of the old site, from the sitemap, from a crawl of the pages and from Search Console reports. Next to each we note how many people it brings and whether other sites link to it. We try not to touch the URLs of pages that bring visitors: the new layout goes onto the old address.
301 redirects
Sometimes a URL has to change: it contains technical parameters, a date or the name of a service that no longer exists. Then we set a 301 redirect, a permanent redirect, from the old URL to the new one. A person arriving from a bookmark or another site lands on the right page, and Google carries over to the new URL what it knew about the old one.
Each old page points to its own matching new page, not everything to the home page. Google advises keeping such redirects for at least a year. We advise never removing them.
Copy and meta tags
We carry over the copy of pages that already rank, without cutting it. If the layout calls for three lines where there are six, we change the layout. The title, description and H1 move with the copy: they aren't visible on a mockup, so they are the easiest thing to lose in a migration.
Search positions
No one can guarantee positions after a redesign. Google crawls the site afresh, and in the first weeks the numbers may swing. We answer for what depends on us: no old URL returns a 404 error, the test copy hasn't got into the index, and analytics and goals work from day one. After launch we watch Search Console reports for two weeks and fix whatever turns up.