Allow for base_font_size_min to be a relative value

[wmat]

Currently, base_font_size_min requires a number, however it would be beneficial to allow for the value to be a relative value such as 0.75em. The documentation for themes here: https://docs.asciidoctor.org/pdf-converter/latest/theme/text/#font-size seems to imply that an em value is allowed, however this is not the case. The following docs seem to conflict with the latter, stating that a number is required: https://docs.asciidoctor.org/pdf-converter/latest/theme/base/#base