HeaderStyle

is an option for Dataset that specifies the style to use for headers.

Details

The setting for HeaderStyle can be any style specification, or a pure function returning the same.
With the option HeaderStylestyle, the specified style will be used for all headers in the dataset.
HeaderStyleAutomatic specifies a style that follows the system appearance for headers.
The settings for HeaderStyle can be given as follows to apply separately to different items:
spec apply spec to all headers

{spec₁,spec₂,…,spec_n} apply spec_k to headers at dataset level k

${spec_(1),spec_(2),...,spec_(n),rules}$ also allow explicit rules for individual headers
The spec_k can have the following forms:

	{s₁,s₂,…,s_n}	use s₁ through s_n; then use defaults
	{{c}}	use c in all cases
	{{c₁,c₂}}	alternate between c₁ and c₂
	{{c₁,c₂,…}}	cycle through all c_i
	{s,{c}}	use s, then repeatedly use c
	{s₁,{c},s_n}	use s₁, then repeatedly use c, but use s_n at the end
	{s₁,s₂,…,{c₁,c₂,…},s_m,…,s_n}	use the first sequence of s_i at the beginning, then cyclically use the c_i, then use the last sequence of s_i at the end
	{s₁,s₂,…,{},s_m,…,s_n}	use the first sequence of s_i at the beginning and the last sequence at the end

Rules have the form ispec, where i specifies a position in the dataset.
Positions can be patterns.
The position of a header can be read at the bottom of a dataset when you hover over the header.
If the left-hand side of a rule is not a list, the setting is applied to any position that contains the left-hand side as a key or index.
A pure function f that returns a style can be used in place of any style. The setting is given by f[item,position,dataset].

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Make all headers bold:

Make headers at successive levels different colors:

Style individual headers:

Style all headers that have a given name:

Use a pattern in a position:

Obtain the header style from a function applied to the header value:

Styles in a list apply to headers at successive levels of the dataset:

To apply multiple styles at a single dataset level, wrap them with Directive:

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	spec	apply spec to all headers
	{spec₁,spec₂,…,spec_n}	apply spec_k to headers at dataset level k
	${spec_(1),spec_(2),...,spec_(n),rules}$	also allow explicit rules for individual headers