org-html-style-include-default

org-html-style-include-default is a variable defined in `ox-html.el'.
Its value is

t

• This variable is an alias for `org-html-head-include-default-style'.
• This variable is obsolete since 24.4;
  use `org-html-head-include-default-style' instead.

Documentation:
Non-nil means include the default style in exported HTML files.
The actual style is defined in `org-html-style-default' and
should not be modified. Use `org-html-head' to use your own
style information.

You can customize this variable.