Fix overriding font-family for both light and dark themes

[federicobond]

The docs mention that the toolbar can be customized by overriding the --djdt-* variables defined in :root, like this:

:root {
    --djdt-font-family-primary: 'Roboto', sans-serif;
}

But with the implementation of dark theme support these same variables are also defined in #djDebug[data-theme="light"] with a higher specificity, so in reality you will need to match the specificity of that rule to override the variables.

This moves the font-family declarations to root so changes to the font-family on :root apply to both light and dark themes.

[matthiask]

Makes sense, thanks!