Changes in the Quick Style gallery on the Home tab, in the Styles group can
be made  just in the active document or can be made in a template (for
example, Normal.dotm(x);

For a paragraph style, linked style or character style to appear in the
Quick Style gallery:
It must have been added to the Quick Style list – the Add to Quick Style
list check box in the Modify Style dialog box (or the Create New Style from
Formatting dialog box) must be checked for that style

One of the following steps must also have been taken:
Click Show after selecting the style in the Recommend tab of the Manage
Styles dialog box (Home tab, Styles group, Styles Dialog Box Launcher, Styles
task pane, Manage Styles)
Click Hide until used after selecting the style in the Recommend tab of the
Manage Styles dialog box and then apply the style in the document or its
parent document template before the document is created

Where a displayed style appears is determined first by its priority value
and, if the priority value of two or more styles is the same, by alphabetical
order.  Click Assign Value on the Recommend tab to display the Assign Value
dialog box (Home tab, Styles group, Styles Dialog Box Launcher, Styles task
pane, Manage Styles)

If you make the changes in a template, you can then update the Quick Style
gallery in a document to which it is attached. Display the Developer tab
(Office button, Word Options; Show Developer tab in Ribbon, OK); on the
Developer tab, in the Templates group, click Document Templates. Click the
Automatically update document styles check box and click OK. Repeat these
steps and uncheck the check box and click OK.