Switch to text view on the editing screen. Then paste the lines of code below and insert your text in the middle.
<div class="govintra-alert govintra-alert--success">
...your block of text...
</div>
The name has two plain hyphens before the colour word; if the box shows no colour, retype the hyphens rather than pasting them from elsewhere.
Use the following text in place of govintra-alert--success to get different colours:
govintra-alert--infogovintra-alert--successgovintra-alert--warninggovintra-alert--error