[BUG]: header un-bolded when themeOverride is used (unless baseFontStyle: 600 13px prop is used)

[walkerlangley]

When setting overrideThemes on a column, it seems that the default header style is removed (no longer bold) and adding the headerFontStyle property does not work. I've set it to bold 13px and 600 13px but neither option works. However, using baseFontStyle: "600 13px" results in both the header and cells being bold (as can been seen in the Theme per Column storybook).