Depending on what kind of border you want, you can use the HTML editor and apply CSS border attributes within the style= attribute.
background, border, clear, color, cursor, direction, display, float, font, height, left line-height, list-style,
margin, max-height, max-width, min-height, min-width, overflow overflow-x, overflow-y, padding, position,
right, text-align, table-layout text-decoration, text-indent, top, vertical-align, visibility, white-space, width
z-index, zoom
That means that you cannot do rounded corners or shadows, but it does support a basic border where you can specify the line thickness, style, and color.
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