In xaringan, how to change the margin under flextable title?

Let's say I have the following xaringan document:

---
title: "Test"
author: 
  - "me"
date: "2022-06-20"
output:
  xaringan::moon_reader:
    css: [default, ninjutsu]
    seal: false
    nature:
      highlightStyle: github
      highlightLines: true
      countIncrementalSlides: false
---
```{r table, tab.cap="Iris", echo=F, warning=F, message=F}
library(tidyverse)
library(flextable)

iris %>%
  head() %>%
  flextable() %>%
  theme_booktabs()
```

which renders like this:

Now I wish to change the spacing between the flextable title and the table itself. What should I do?

I have no idea where the settings to be changed are defined, but when changing them, the extra space disappears... It needs to be done in css and the properties to set to 0 are 'margin-block-start' and 'margin-block-end'.

---
title: "Test"
author: 
  - "me"
date: "2022-06-20"
output:
  xaringan::moon_reader:
    css: [default, ninjutsu]
    seal: false
    nature:
      highlightStyle: github
      highlightLines: true
      countIncrementalSlides: false
---

```{css}
p {
  margin-block-start: 0px;
  margin-block-end: 0px;
}
```


```{r table, tab.cap="Iris", echo=F, warning=F, message=F}
library(tidyverse)
library(flextable)

iris %>%
  head() %>%
  flextable() %>%
  theme_booktabs()
```