@sebaseba - You’re looking for the “class name” of the objects you want to target for styling. Here’s a way that works for me:
.HyperMD-footnote, .cm-hmd-footnote {color:var(--text-muted);}
If you don’t like the color shown, change the var(--text-muted) part to any valid variable or color (hex, rgb, etc.) of your choice. The first bit (.HyperMD-footnote) is the class for the footnote text, the second (.cm-hmd-footnote) targets the part in brackets.
Also, I notice a lot of theme authors are overly liberal with the use if important flags, so you may need to add that if the above doesn’t work:
.HyperMD-footnote, .cm-hmd-footnote {color:var(--text-muted) !important;}
Try not to use the important flag unless it’s absolutely necessary. It makes future styling difficult 
Hope that helps.