Thank you for your advice. However, only this code has solved the issue:

@font-face {
  font-family: FivoSans;
  font-style: normal;
  src:
    url("FivoSans-Medium.woff2") format("woff2"),
    url("FivoSans-Medium.woff") format("woff");
}
.fivosans-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}

.fivosans-medium-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}
.newsfeed .medium {
  font-family: FivoSans;
}

I had to add .medium, and now it works Thank you very much!

Hi, thank you for your time. However, it hasn’t helped. Maybe there is another problem in my CSS file?

Hi guys,

I would like to inquire you to help me with changing a newsfeed font.
I`ve already done this modification of custom.css:

@font-face {
  font-family: FivoSans;
  font-style: normal;
  font-weight: medium;
  src:
    local("Fivo"),
    url("Fivo/FivoSans-Medium.woff2") format("woff2"),
    url("Fivo/FivoSans-Medium.woff") format("woff");
}
.fivosans-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}

.fivosans-medium-custom-class {
  font-family: 'FivoSans';
  font-weight: medium;
}
.newsfeed {
  font-family: FivoSans;
}

body {
}  

The font was converted from TrueType to Woff and Woff2 a I have copied it into this folder:

/home/pi/MagicMirror/fonts/node_modules/roboto-fontface/fonts/Fivo

I am a Newbie in CSS, so if you have any idea what I do wrong, give me an advice, please.