Source

beamer / beamerthemeplain.sty

Diff from to

File beamerthemeplain.sty

                                 % Frametitles
 \useframetitletemplate{%
   \vskip1.5em
-  {\leftskip-\Geom@lmargin plus1fill\advance\leftskip by0.3cm%
-   \rightskip-\Geom@rmargin plus1fill\advance\rightskip by0.3cm\leavevmode
+  {\leftskip-\beamer@leftmargin plus1fill\advance\leftskip by0.3cm%
+   \rightskip-\beamer@rightmargin plus1fill\advance\rightskip by0.3cm\leavevmode
     \color{structure}\Large\textrm{\textit{{\insertframetitle}}}\par}
   }