A function to tweak minor detail of other themes

[gadenbuie]

Currently, xaringanthemer writes a complete version of the default xaringan theme, but it occurred to me that, now that there are more user-contributed themes available, users may want to modify or tweak a small portion of another theme. For example, a user may want to use the metropolis theme but change font sizes or colors.

I could provide a function, e.g. modify_theme(), that writes inline CSS into the slide deck to modify small aspects of the remark theme. For example, changing the header font size might be done with

```{r echo=FALSE}
xaringanthemer::modify_theme(header_h1_font_size = "24px")
```

<style>
.remark-slide-content h1 {
  font-size: 24px;
}
</style>

Technically speaking this may be easier to do now that I rewrote the template in mustache.

If you're interesting in this feature, an thumbs up on this issue will be very motivating! 😉