color: turn colorui functions into forwards when color is None

colorui will be set to None as necessary in an upcoming patch.

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branch : stable
color: set _colormode to None when mode is unset (issue3895)

Previously, colorui assumed that it would only be called when mode wasn't
None. 7ae12ce87594 changed that, so now colorui needs to care about whether it
should colorize output.

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branch : stable
color: add a test with extension loaded and ui.formatted=False

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branch : stable
css: fixed font-family

There is no 'sans' font-family, replaced with 'sans-serif'

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branch : stable
css: remove repeated property

'margin' property was repeated for the same selector

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branch : stable