Please, have a look at the app: https://apps.odoo.com/apps/modules/8.0/colors_customization/

• If you are asking for Home Login Page, follow below link
  • https://www.odoo.com/forum/how-to/miscellaneous-14/how-to-remove-manage-databases-2615
• You may want to check similar ways as above to modify from other template pages

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If you are asking for reports to be modified then follow below -

Header / Footer option is not shown by default, activate it by going to Settings > Users > Activate Technical Features... 

Then Just edit the Company Configuration inside openerp by clicking on your company Logo. Select Header/Footer and there you will find the code :

            <!-- left margin -->
            <rotate degrees="90"/>
            <fill color="grey"/>
            <drawString x="2.65cm" y="-0.4cm">Powered by OpenERP</drawString>
            <fill color="black"/>
            <rotate degrees="-90"/>

I am sorry i wrongly assumed you were talking about OpenERP mentioned in footer of reports - Quotations/Sales Orders.