[theme-reviewers] Guidelines when revamping existing parent theme or framework

[Mario Peshev]

Hello Cais,

I don't argue with the Theme Review team or the guidelines authors - it is
just the third time I encounter a similar case and I thought it would be
good to define what is a minor and what - a major and significant change (so
to speak) so that it is clear that changing a tiny font/background element
doesn't do a theme uniqueness.