Does changing Strong element CSS styling affect SEO??
Two conflicting versions of the original research paper by Brin and Page at Stanford University, describes Google before it even began. The experimental/research version of Google (still exists by the way - the last time I looked anyway) laid the foundation for the commercial product.
In these documents, one describes various styling as being fancy to be indicated within the index. These include mark-up such as bold, italics, capitalization, etc. This is before CSS (cascading style sheets) when HTML coding was still fairly basic. These indications were for single terms within the index and stored in a binary format to indicate what kind of fancy was found for the term.
One thing that Brin and Page emphasized constantly over the years, is that any stored data must have significance. The reason for this is simple. Hard drives were and remain expensive and relatively small and therefore indexing capacity was limited though much storage potential has raised significantly over they years. It remains true that storage is a premium. While it was not made clear that I can see, indications that I can find seems to elude that some of these fancy indications were found to be statistically insignificant and may have been dropped as this binary storage technique has changed and been expanded over the years.
Keep in mind that from the very beginning, Google was intended to be a semantics search engine and does not make direct term matches but rather attempts to match search intent using semantic analysis. This has been significantly expanded to near full capacity with major leaps in semantic capability beginning in 2002, 2003, 2005, and 2008, and continuing. As part of semantic analysis, fancy does not apply though it can still be used. However, semantic analysis of HTML uses the DOM model and fancy does not fit this model.
Was/Is there a penalty for style HTML mark-up??
No. Or at least only temporarily in limited forms.
Keep in mind, most SEOs are not technical people and glom onto anything they think may be significant or an advantage.
At some point, it was decided that bold and italics amongst other forms of fancy contained an advantage, however, this was not true at the time. As a result, many sites began to try and take advantage of this. I do not believe that there ever was a specific penalty for this behavior, however, fancy was at least ignored and sites that attempted to abuse this ideology, may have been singled out as spam through manual means or other forms of analysis.
Now that CSS exists and little HTML mark-up contains fancy, the complexity has been greatly expanded. I do not believe, in light of the emphasis upon semantic analysis, that fancy was ever expanded or emphasized to include styling mark-up. Semantics is about content analysis and not style.
So does style matter for matching search intent to content??
No. At least not for semantic analysis.
While style does matter for user experience, fancy remains completely ignored for matching search intent to content. This is evidenced in how SERP snippets that are not taken from the description meta-tag are taken and presented. You will not find matches for content marked with style as an indication nor will you find header tags and other similar mark-up. You will find that straight content often found in paragraphs, blockquotes, tables, etc., are used.