IPython/html/static/notebook/less/cell.less

.cell {
    border: 1px solid transparent;
    .vbox();

    &.selected {
        .corner-all;
        border : thin @border_color solid;
    }
}

div.cell {
    width: 100%;
    padding: 5px 5px 5px 0px;
    /* This acts as a spacer between cells, that is outside the border */
    margin: 2px 0px 2px 7px;
    outline: none;
}

div.prompt {
    /* This needs to be wide enough for 3 digit prompt numbers: In[100]: */
    width: 11ex;
    /* This 0.4em is tuned to match the padding on the CodeMirror editor. */
    padding: 0.4em;
    margin: 0px;
    font-family: @monoFontFamily;
    text-align: right;
    /* This has to match that of the the CodeMirror class line-height below */	
    line-height: @code_line_height;
}