If user config file overwrites a single attribute and not the others,
for example only bg:
"mail.listing.tag_default" = { bg = "Blue" }
The other attributes, in this case fg and attrs revert to the default
values of ThemeAttributeInner and not the default value for the key
"mail.listing.tag_default". As a result the above expands to:
"mail.listing.tag_default" = { fg = Color::Default, bg = "Blue", attrs
= Attr::Default }
This commit keeps the key value defaults, so the above should expand to:
"mail.listing.tag_default" = { fg = default_theme["mail.listing.tag_default"].fg, bg = "Blue", attrs
= default_theme["mail.listing.tag_default"].attrs }