What does theme-modifier mean in BEM?

Question

BEM

In the description, BEM encountered the fact that sometimes a modifier with a theme name is applied ... and what properties does this modifier usually contain?

for example

<form class="form  form_size_m form_color_red_form_theme_forest"></form>

form_size_m sets the font size, form_color_red sets the text color to red, and _form_theme_forest sets what? It's not a position or a color