Using the CTRL button to select multiple text segments in Design Editor

No it doesn't work that way. What you need to do is create some character styles in your style sheet and apply those.

Paragraph styles are defined something like

P {
    font-size: 10pt;
    margin-top: 0pt;
    margin-bottom: 6pt;
    font-family: Verdana, sans-serif;
    color: #000000;
    font-weight: normal;
}
P.Normal-Indent {
    margin-left: 20pt;

}

where the second P definition will inherit all that is in P plus add the indent.

When you select those styles, they will be applied to the whole paragraph.

You can also create characters classes. If you do it within RoboHelp, RH will write the CSS code for you. If you edit the CSS outside RH, then instead of writing P or some other tag, you would normally start with the full stop so you might have

.code {
    font-size: xx-small;
    font-family: Courier;
}

to apply the Courier font to a selection in any paragraph style.

In RoboHelp you need to prefix the character code with "span" to make it display in the dropdown, thus you would need:

span.code {
    font-size: xx-small;
    font-family: Courier;
}

See www.grainge.org for RoboHelp and Authoring tips