Twenty Fifteen: Add a check if the custom color is the default of the chosen color scheme.

[iamtakashi]

Currently, twentyfifteen_header_background_color_css() and twentyfifteen_sidebar_text_color_css() only check if the value is the same as the default color of the default color scheme #ffffff, #333333. This causes duplicated styles in the head when the color scheme is not the default. Also this breaks if you choose #ffffff for the sidebar color even your color scheme is dark, for example.

[iamtakashi]

Add a check if the custom color is the default of chosen color scheme.

[iandstewart]

In 30221:

Twenty Fifteen: check if user-selected custom colors are the same as default so we're not duplicating styles.

Props iamtakashi, fixes #30234