Slidify - diacritic marks issue: different title slide font in RStudio Knit HTML view and slidify html output

Question

I am developing a presentation with the use of R slidify function. In my presentation I use diacritic marks from Polish language (Ą,Ć,Ę,Ł,Ń,Ó,Ś,Ź,Ż). I have noticed that their display (only in case of the presentation title slide) does differ depending on whether I render its html view via Knit HTML button in RStudio or I view the html file output generated by the slifidy function.

This is a screen from a "Knit HTML button view" presentation title slide:

This is a screen from an html file output generated by the slifidy function (in which I additionally marked some of outstandingly displayed marks):

Question: why does such difference in a font type occur and, more important, how to avoid this? (Please note it happens only in the title slide.)

My presentation begins with the structure:

---
title       : Mąlarądóałatrią o (ą,ć,ę,ł,ń,ó,ś,ź,ż ; Ą,Ć,Ę,Ł,Ń,Ó,Ś,Ź,Ż) 
subtitle    : Podtytuł mąlarądóałatrią o (ą,ć,ę,ł,ń,ó,ś,ź,ż ; Ą,Ć,Ę,Ł,Ń,Ó,Ś,Ź,Ż) 
author      : Imię Nazwisko 
job         : Pisanie prezentacji o (ą,ć,ę,ł,ń,ó,ś,ź,ż ; Ą,Ć,Ę,Ł,Ń,Ó,Ś,Ź,Ż) 
framework   : io2012        # {io2012, html5slides, shower, dzslides, ...}
highlighter : highlight.js  # {highlight.js, prettify, highlight}
hitheme     : tomorrow      # 
widgets     : []            # {mathjax, quiz, bootstrap}
mode        : selfcontained # {standalone, draft}
---

## Wstęp o ą,ć,ę,ł,ń,ó,ś,ź,ż ; Ą,Ć,Ę,Ł,Ń,Ó,Ś,Ź,Ż

[...] 

Answer 1

As I wrote in the comments below my question, I eventually decided to solve this problem by modifying the .css file: \mydeck\libraries\frameworks\io2012\css\slidify.css.

Here is the default .css file I receive after first slidify over a document, and here you can find customized .css file which deals with the font type inconsistency. (Of course, in the slidify files directory it is going to exist under "old" slidify.css name.) The only changes I applied are these under

/* Styles for Title Slide */

section.