[dspace-source]/dspace-xmlui-mirage2/src/main/webapp/xsl/core/page-structure.xml
and place it in at:
[dspace-source]/dspace/modules/xmlui-mirage2/src/main/webapp/themes/xsl/core/page-structure.xml
Now on the copied file around line 100 you will see this div, this is the one that outputs your content and the sidebar
<div id="main-container" class="container">
<div class="row row-offcanvas row-offcanvas-right">
<div class="horizontal-slider clearfix">
<div class="col-xs-12 col-sm-12 col-md-9 main-content">
<xsl:apply-templates select="*[not(self::dri:options)]"/>
<div class="visible-xs visible-sm">
<xsl:call-template name="buildFooter"/>
</div>
</div>
<div class="col-xs-6 col-sm-3 sidebar-offcanvas" id="sidebar" role="navigation">
<xsl:apply-templates select="dri:options"/>
</div>
</div>
</div>
At around line 111, you will get this code which is responsible for the sidebar
<div class="col-xs-6 col-sm-3 sidebar-offcanvas" id="sidebar" role="navigation">
<xsl:apply-templates select="dri:options"/>
</div>
Comment out the line that imports
Navigation.xsl.
Then recompile.