To make a Standard style the default style for a new created drawing document you have to pre-define this in the template file, i.e. you can do this as below:
1. Make sure the Use Style Library is Read-Write for current project.
2. Open the drawing template(usually the ...\Autodesk\Inventor 2013\Templates\Standard.idw).
3. Go to the Style and Standard Editor, and activate the Standard style(like Samsul Standard), and then click the Object Defaults tab, and find the Active Object Defaults style, and click the edit button.
4. Now in the Object Defaults style, change an Object Style for an Object Type(like you can change the Linear Dimension Object Style to your Samsul Dim).
5. Save changes to the styles, and also make you right-click the styles(Dimension, Object Defaults, Standard styles) that you just made changes and choose Save to Style Library.
6. Save the template document.
7. Now new a drawing document based on the template and verify that the standard and linear dimension style is now using yours.
For other styles you can follow above steps to make yours default styles in the drawing template.
Hope this helps.
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Rocky Zhang
Inventor API PD
Manufacturing Solutions
Autodesk, Inc.