The cell_borders() helper function is to be used with the tab_style()
function, which itself allows for the setting of custom styles to one or more
cells. Specifically, the call to cell_borders() should be bound to the
styles argument of tab_style(). The selection argument is where we
define which borders should be modified (e.g., "left", "right", etc.).
With that selection, the color, style, and weight of the selected
borders can then be modified.
cell_borders(sides = "all", color = "#000000", style = "solid", weight = px(1))A list object of class cell_styles.
The border sides to be modified. Options include "left",
"right", "top", and "bottom". For all borders surrounding the
selected cells, we can use the "all" option.
The border color, style, and weight. The color
can be defined with a color name or with a hexadecimal color code. The
default color value is "#000000" (black). The style can be one of
either "solid" (the default), "dashed", "dotted", or "hidden".
The weight of the border lines is to be given in pixel values (the
px() helper function is useful for this. The default value for weight
is "1px". Borders for any defined sides can be removed by supplying
NULL to any of color, style, or weight.
Add horizontal border lines for all table body rows in exibble using
tab_style() and cell_borders().
exibble %>%
gt() %>%
tab_options(row.striping.include_table_body = FALSE) %>%
tab_style(
style = cell_borders(
sides = c("top", "bottom"),
color = "red",
weight = px(1.5),
style = "solid"
),
locations = cells_body(
columns = everything(),
rows = everything()
)
)

Incorporate different horizontal and vertical borders at several locations.
This uses multiple cell_borders() and cells_body() calls within
list()s.
exibble %>%
gt() %>%
tab_style(
style = list(
cell_borders(
sides = c("top", "bottom"),
color = "#FF0000",
weight = px(2)
),
cell_borders(
sides = c("left", "right"),
color = "#0000FF",
weight = px(2)
)
),
locations = list(
cells_body(
columns = num,
rows = is.na(num)
),
cells_body(
columns = currency,
rows = is.na(currency)
)
)
)

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Other helper functions:
adjust_luminance(),
cell_fill(),
cell_text(),
cells_body(),
cells_column_labels(),
cells_column_spanners(),
cells_footnotes(),
cells_grand_summary(),
cells_row_groups(),
cells_source_notes(),
cells_stub_grand_summary(),
cells_stub_summary(),
cells_stubhead(),
cells_stub(),
cells_summary(),
cells_title(),
currency(),
default_fonts(),
escape_latex(),
google_font(),
gt_latex_dependencies(),
html(),
md(),
pct(),
px(),
random_id(),
stub()