Use the same button style and color what LO uses for header/footer
button.
Later it might be a good idea to have a separate configurable color
for this button, but for now it's seems unneeded to introduce a
new configure option for this.

Change-Id: Idf86aaa44e29846a2ee211a81fe5376b4335bb05
Reviewed-on: https://gerrit.libreoffice.org/65818
Tested-by: Jenkins
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>