This vignette cover some basic CSS, in particular to give an idea about the possible values you can use with the various arguments to the xaringan theme functions. See vignette("template-variables", package = "xaringanthemer") for a full list of the theme functions.

Because we are setting the CSS properties from R, we can either give xaringanthemer function arguments a character string or we can call an R function or variable that returns a character string. For example, we can create an R variable with a specific color that is used in several places in a theme

firebrick <- "#CD2626"
write_xaringan_theme(
)

or we can directly give the character string

write_xaringan_theme(
)

in both cases, we get CSS like the following that sets the link color

a, a > code {
color: #CD2626;
}

Note that when a string is given to the theme function, the outer quotes are removed.

In the sections below, R code is represented without quotes – like rgb(0.8, 0.15, 0.15) – and CSS code is represented inside quotes – like "rgb(205, 38, 38)" – to differentiate between R and CSS functions with the same or similar names.

## Colors

In CSS, text colors are specified with the color: property, background colors use background-color:, and border colors use border-color:.

In xaringanthemer, template variables that set

• text color end with _color;
• background color end with _background_color;
• border color end with _border_color;

### Setting colors

In CSS, there are a number of ways to specify a color:

• You can use a color keyword, such as "darkslategray" or "red".

• You can use the RGB color specification either via

• "#CD2626" or
• "#CD262680" (50% transparency)
• or the rgb function notation
• "rgb(255, 38, 38)" or
• "rgba(255, 38, 28, 0.5)" (50% transparency).
• You can use the HSL color specification via the functions

• "hsl(270,60%,70%)" or
• "hsl(270, 60%, 50%, .15)" (15% transparency).

In R, there are a number of ways to specify a color:

• Use rgb(205, 38, 38, maxColorValue = 255) as an equivalent to the CSS "rgb()" function.

• Without the maxColorValue argument, the rgb() function expects decimal numbers in the range [0, 1], like rgb(0.8039, 0.1490, 0.1490).
• The rgb() function also sets transparency via the alpha argument (in the [0, 1] range).
• You can get the hexadecimal representation of a built-in R color using the col2rgb() function together with the rgb() function:

rgb(t(col2rgb("red")), maxColorValue = 255)
#> [1] "#FF0000"

## Sizes

In xaringanthemer, any template variable that accepts a CSS size (or length unit) ends with _size. Sizes are also used for positioning and those template variables end include position in their name.

There are many units available in CSS sizes, but the three most common and easiest to use are pixels ("px"), percentage ("%"), and em units ("em"). Mozilla’s devloper portal has a full list of CSS length units.

These sizes are either absolute or relative values. Relative values are set relative to the size of the parent element, but absolute values ignore the parent element.

Pixels "px"
Pixels are an absolute size unit, traditionally representing one device pixel. E.g. "16px" or "23px".
Percentage "%"
Percentages are relative to the size of the parent element, scaled linearly. E.g. "75%" or "150%".
em Units "em"
em Units are just like percentages, except expressed as decimals. E.g. "0.75em" or "1.5em".

To make this more concrete, here is a simple “page” containing a section header and two paragraphs.

<div class="page">
<h1>Section 1</h1>
<p>This is paragraph 1...</p>
<p>This is paragraph 2...</p>
</div>

Intuitively, you might want the section header to have a somewhat bigger font size than the paragraph text, but you don’t want to have to set the text size for each and every paragraph or header.

To do this, we can set the base size of any element inside the <div class="page">, and adjust the header size relatively.

.page {
font-size: 16px;
}
h1 {
font-size: 2em;
}

Now our paragraph font will be 16 pixels tall, and the level 1 headers will be twice as big. If we later decide to change the base font size, say to "15px", the header text will still be twice as big as the paragraph text.

## Positioning

If you’re reading this, you’re probably wondering how you make an element be where you want it to be.

• background_position (background image position)
• title_slide_background_position (title slide background image position)
• footnote_position_bottom (footnote location from bottom of screen)

### Footnote Position

xaringanthemer provides one template variable to adjust the position of the footnote element. Footnotes can be insterted into a slide using the .footnote[Here's my quick footnote] syntax.

The footnote_position_bottom argument adjust how far from the bottom of the slide the footnote appears. The default value is "3em", but you can adjust this value up or down to get the footnote where you want.

### Background Position

The background position is set using the theme function arguments that end with background_position. See this article on background-position from Mozilla for more information.

Try any of the following values to get started:

background_position = "center"
background_position = "top"
background_position = "left"
background_position = "bottom"
background_position = "25% 75%" # X-value (from left) Y-value (from top)
background_position = "bottom 10px right 20px" # 10px from bottom, 20px from right
background_position = "top left 10px" # at top but adjusted left 10px

### General Positioning

Read this section if you want to put a slide element at a specific spot on your slide.

The position CSS element is used to specify where an element is located on the screen. Mozilla provides a very good reference on positioning that I’ve summarized here.

An element can be "relative"-ly positioned, "absolute"-ly positioned, "fixed", "sticky" or "static" (default). For an element with a computed position (i.e. not "static"), you can also specify the top, right, bottom, and left CSS properties for that element. The top/bottom parameters specify vertical displacement, and the right/left specify horizontal displacement.

Relatively positioned position: relative
For relatively positioned elements, the element position is adjusted relative to where it would have been if it were static.
Absolutely positioned position: absolute or position: fixed
Absolutely positioned elements are positioned relative to the block that contains the element (called a containing block). A fixed element won’t move with scrolling (but fixed is not recommended for remarkjs slides).

If you want something to appear in a specific position on your slide, you’ll need to use the extra_css argument of the xaringanthemer functions. For example, lets say you want a 300px by 300px box to appear on the right side of your slide, you’ll need to create a special css class:

write_xaringan_theme(
extra_css = list(
".box-right" = list(
"height" = "300px",
"width" = "300px",
"position" = "absolute",
"top" = "33%",
"left" = "65%"
)
)
)

This creates CSS like this:

.box-right {
height: 300px;
width: 300px;
position: absolute;
top: 33%;
left: 65%;
}

which you can then use in your slides by wrapping the slide content in .box-right[].

.box-right[
Stuff inside the box
]