Spaces between columns in stargazer type = "html" table output

You can also drop the CSS directly into the RMarkdown document (see here). eg.

---
title: "Untitled"
author: "Author"
date: "29 June 2017"
output: html_document
---

```{css, echo = FALSE}

table, td, th {
  border: none;
  padding-left: 1em;
  padding-right: 1em;
  margin-left: auto;
  margin-right: auto;
  margin-top: 1em;
  margin-bottom: 1em;
}

```


```{r, results = "asis"}

stargazer::stargazer(mtcars, type = "html")

```

If you are writing an RMarkdown document, you can customize your HTML tables using stylesheets.

This can be done adding the option CSS to your YAML header. Something like this:

---
title: "My HTML Doc"
output:
  html_document:
    css: styles.css
---

To increase the spacing between columns you could, for example, add some padding to the left of the cells. So, in your styles.css files you could put something like:

th, td {
    padding-left: 10px;
    text-align: left;        
}

For further information about using CSS in RMarkdown, please check this. For more about CSS for HTML tables, please check this.