unify the pdf theme that all specs use

[kbroch-rivosinc]

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://github.com/riscv/riscv-isa-manual/issues/1448