Fixed accidentally commented out .sd-header subsequent issues

[yaosarayin]

1. Fixed 4-row header: .sd-header { display: flex; } is no longer commented out, so
   the left and right columns sit side by side as intended
2. Fixed light mode colors not working background: var(--sd-header-bg) and color:
   var(--sd-header-color) now apply, so the UW purple (#460084) and white text from
   slowdash-light.css take effect
3. Fixed default to white background - default theme is 'light',
   slowdash-light.css is always loaded and sets --sd-header-bg: #460084.
   .sd-header rule now active, that purple background is applied even when no style is
   specified in SlowdashProject.yaml

[SanshiroEnomoto]

Thanks for the quick fix!