unify the pdf theme that all specs use

This is just the isa-manual riscv-spec.yml copied to the riscv-pdf.yml
fixes https://github.com/riscv-admin/docs-sig/issues/48
relates to doing this next: https://www.npmjs.com/package/yaml-sort



Signed-off-by: Kevin Broch <kbroch@rivosinc.com>