if you don't want to have the three columns in desktop view, simply create a user.css in selfoss's homedir with the following:

/* Inhalte nicht in 3 Spalten anzeigen */

.entry-content {
display:none;
-moz-column-count: 1;
-moz-column-gap: 2em;
-moz-column-rule: 1px solid rgba(204, 204, 204, 0.4);
-webkit-column-count: 1;
-webkit-column-gap: 2em;
-webkit-column-rule: 1px solid rgba(204, 204, 204, 0.4);
column-count: 1;
column-gap: 2em;
column-rule: 1px solid rgba(204, 204, 204, 0.4);
font-size:0.95em;
margin-top:20px;
line-height: 1.7em;
}

Delete the file public/all.css if it is there.

Refresh your browser.

Greets
Mario