Ok sorted:

in Users/DetailView.ascx add above the row that has LBL_LANGUAGE (around line 100):

<tr>
   <td class="tabDetailViewDL"><%= L10n.Term("Users.LBL_THEME") %>&nbsp;</td>
   <td class="tabDetailViewDF"><asp:Label ID="txtTHEME" Runat="server" /></td>
   <td class="tabDetailViewDF"><%= L10n.Term("Users.LBL_THEME_TEXT") %>&nbsp;</td>
</tr>

in Users/EditView.acsx around line 98 add:

AutoPostBack="true"

to the Theme dropdown

Now the tricky bit... in Users/DetailView.ascx.cs add the following just under //User Settings at around line 250:

// hacked by SP by copying the Language one and changing the fields

txtTHEME.Text = L10N.NormalizeCulture(XmlUtil.SelectSingleNode(xml, "theme"));

try

DataView vwThemes = new DataView(SplendidCache.Themes());

vwThemes.RowFilter = "NAME = '" + txtTHEME.Text + "'";

if (vwThemes.Count > 0)

txtLANGUAGE.Text = Sql.ToString(vwThemes[0]["THEME_NAME"]);

catch (Exception ex)

SplendidError.SystemWarning(new StackTrace(true).GetFrame(0), ex.Message);

// end of hack