Add CSS variable for table header text color

[Jose-Verdu-Diaz]

Furo currently offers a CSS variable for modifying the background colour of table headers:

furo/src/furo/assets/styles/variables/_colors.scss

Lines 76 to 78 in 0439e84

	// Tables
	--color-table-header-background: var(--color-background-secondary);
	--color-table-border: var(--color-background-border);

Currently, the table header text colour is set to --color-foreground-primary, however, this may lead to problematic colour combinations when modifying the header background colour.

It would come handy to add a new variable --color-table-header-text as follows:

--color-table-header-text: var(--color-foreground-primary);

In this way, the table header text could be overwritten from the sphinx config file.

[pradyunsg]

#818 filed for this.