

Overlays are like extensions in that you tell Elementary that some other theme contains the styles you need for your program. The difference is that they will be look in first, so they can override the default style of any widget.

There\textquotesingle{}s not much to say about them that hasn\textquotesingle{}t been said in our previous example about \mbox{\hyperlink{theme_example_01}{extensions}}, so going quickly through the code we have a function to load or unload the theme, which will be called when we click any button. 
\begin{DoxyCodeInclude}{0}
\DoxyCodeLine{\textcolor{preprocessor}{\#include <Elementary.h>}}
\DoxyCodeLine{\textcolor{keyword}{static} \textcolor{keywordtype}{void}}
\DoxyCodeLine{\_btn\_clicked\_cb(\textcolor{keywordtype}{void} *data \mbox{\hyperlink{group__Eina__Types__Group_ga82d751be730f849719ca292f9558be87}{EINA\_UNUSED}}, \mbox{\hyperlink{group__Evas__Object__Group_ga9e19e6dd1f517a0ba437c0114d3e7c97}{Evas\_Object}} *obj \mbox{\hyperlink{group__Eina__Types__Group_ga82d751be730f849719ca292f9558be87}{EINA\_UNUSED}}, \textcolor{keywordtype}{void} *ev \mbox{\hyperlink{group__Eina__Types__Group_ga82d751be730f849719ca292f9558be87}{EINA\_UNUSED}})}
\DoxyCodeLine{\{}
\DoxyCodeLine{   \textcolor{keyword}{static} \textcolor{keywordtype}{int} loaded = 1;}
\DoxyCodeLine{   \textcolor{keywordflow}{if} (loaded)}
\DoxyCodeLine{     \mbox{\hyperlink{group__Elm__Theme_ga100c2a0f541686f3272cc6009aae5864}{elm\_theme\_overlay\_del}}(NULL, edj\_path);}
\DoxyCodeLine{   \textcolor{keywordflow}{else}}
\DoxyCodeLine{     \mbox{\hyperlink{group__Elm__Theme_ga6c9e97741f3893e14508ec5e4410b4f0}{elm\_theme\_overlay\_add}}(NULL, edj\_path);}
\DoxyCodeLine{   loaded = 1 -\/ loaded;}
\DoxyCodeLine{\}}

\end{DoxyCodeInclude}
 And the main function, creating the window and adding some buttons to it. We load our theme as an overlay and nothing else. Notice there\textquotesingle{}s no style set for any button there, which means they should be using the default that we override. 
\begin{DoxyCodeInclude}{0}
\DoxyCodeLine{EAPI\_MAIN \textcolor{keywordtype}{int}}
\DoxyCodeLine{elm\_main(\textcolor{keywordtype}{int} argc \mbox{\hyperlink{group__Eina__Types__Group_ga82d751be730f849719ca292f9558be87}{EINA\_UNUSED}}, \textcolor{keywordtype}{char} **argv \mbox{\hyperlink{group__Eina__Types__Group_ga82d751be730f849719ca292f9558be87}{EINA\_UNUSED}})}
\DoxyCodeLine{\{}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group_ga9e19e6dd1f517a0ba437c0114d3e7c97}{Evas\_Object}} *win, *box, *btn;}
\DoxyCodeLine{}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__General_ga7a05f37fd3a019da5e26c5fc91fcd937}{elm\_policy\_set}}(\mbox{\hyperlink{group__Elm__General_ggaa16f3378ecdbf660fa378402e3c5a0a9a02fadb6faada04b19d0fa42c11a8d32a}{ELM\_POLICY\_QUIT}}, \mbox{\hyperlink{group__Elm__General_gga46af37e6ac2c406321060be827117196abbe0962d88ae12afe1a173da881f56cd}{ELM\_POLICY\_QUIT\_LAST\_WINDOW\_CLOSED}});}
\DoxyCodeLine{}
\DoxyCodeLine{\textcolor{preprocessor}{\#ifdef PACKAGE\_DATA\_DIR}}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__App_ga4e875d27efb02b4c425619957c0ae7c7}{elm\_app\_compile\_data\_dir\_set}}(PACKAGE\_DATA\_DIR);}
\DoxyCodeLine{\textcolor{preprocessor}{\#endif}}
\DoxyCodeLine{   \textcolor{keywordflow}{if} (\mbox{\hyperlink{group__Ecore__File__Group_gaf5a3cc8bd9bf69a3ebce0678fe799016}{ecore\_file\_exists}}(\textcolor{stringliteral}{"{}./theme\_example.edj"{}}))}
\DoxyCodeLine{     \{}
\DoxyCodeLine{        strcpy(edj\_path, \textcolor{stringliteral}{"{}./theme\_example.edj"{}});}
\DoxyCodeLine{     \}}
\DoxyCodeLine{   \textcolor{keywordflow}{else}}
\DoxyCodeLine{     \{}
\DoxyCodeLine{        \mbox{\hyperlink{group__Elm__App_ga0cefb59efb95841232561d67e34d9909}{elm\_app\_info\_set}}(elm\_main, \textcolor{stringliteral}{"{}elementary"{}}, \textcolor{stringliteral}{"{}examples/theme\_example.edj"{}});}
\DoxyCodeLine{        snprintf(edj\_path, \textcolor{keyword}{sizeof}(edj\_path), \textcolor{stringliteral}{"{}\%s/examples/theme\_example.edj"{}},}
\DoxyCodeLine{                 \mbox{\hyperlink{group__Elm__App_ga92a09dbf88cc1ca3eab51c51dea589d0}{elm\_app\_data\_dir\_get}}());}
\DoxyCodeLine{     \}}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Theme_ga6c9e97741f3893e14508ec5e4410b4f0}{elm\_theme\_overlay\_add}}(NULL, edj\_path);}
\DoxyCodeLine{}
\DoxyCodeLine{   win = \mbox{\hyperlink{group__Elm__Win__Group_gac75d7752662fab73d6420706ce5be996}{elm\_win\_util\_standard\_add}}(\textcolor{stringliteral}{"{}theme"{}}, \textcolor{stringliteral}{"{}Theme example"{}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Win__Group_ga9a16a2f8c9fd5b6e9203ed9dcb53ea8b}{elm\_win\_autodel\_set}}(win, \mbox{\hyperlink{group__Eina__Types__Group_ga05c12dacc8b4058994df842b41be85fc}{EINA\_TRUE}});}
\DoxyCodeLine{}
\DoxyCodeLine{   box = \mbox{\hyperlink{group__Elm__Box__Group_ga3d0998b60ada633bacfa8869a921cfd7}{elm\_box\_add}}(win);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group_ga2af897b00f32bebf08705929276386f5}{evas\_object\_size\_hint\_weight\_set}}(box, \mbox{\hyperlink{Evas__Common_8h_aa47c85083d3dae0111c21ec49359306a}{EVAS\_HINT\_EXPAND}}, \mbox{\hyperlink{Evas__Common_8h_aa47c85083d3dae0111c21ec49359306a}{EVAS\_HINT\_EXPAND}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Win__Group_ga505d6bd217e5ee01e82b548ec35986eb}{elm\_win\_resize\_object\_add}}(win, box);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(box);}
\DoxyCodeLine{}
\DoxyCodeLine{   btn = \mbox{\hyperlink{group__Elm__Button_ga0021b9d86d822367035843ed18bc6ea9}{elm\_button\_add}}(win);}
\DoxyCodeLine{   elm\_object\_text\_set(btn, \textcolor{stringliteral}{"{}Button 1"{}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Box__Group_ga66a10dd861578521af2ab7b8c2ca65c5}{elm\_box\_pack\_end}}(box, btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Smart__Object__Group_ga18a8b179f94d21b2b09e19db11741061}{evas\_object\_smart\_callback\_add}}(btn, \textcolor{stringliteral}{"{}clicked"{}}, \_btn\_clicked\_cb, NULL);}
\DoxyCodeLine{}
\DoxyCodeLine{   btn = \mbox{\hyperlink{group__Elm__Button_ga0021b9d86d822367035843ed18bc6ea9}{elm\_button\_add}}(win);}
\DoxyCodeLine{   elm\_object\_text\_set(btn, \textcolor{stringliteral}{"{}Button 2"{}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Box__Group_ga66a10dd861578521af2ab7b8c2ca65c5}{elm\_box\_pack\_end}}(box, btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Smart__Object__Group_ga18a8b179f94d21b2b09e19db11741061}{evas\_object\_smart\_callback\_add}}(btn, \textcolor{stringliteral}{"{}clicked"{}}, \_btn\_clicked\_cb, NULL);}
\DoxyCodeLine{}
\DoxyCodeLine{   btn = \mbox{\hyperlink{group__Elm__Button_ga0021b9d86d822367035843ed18bc6ea9}{elm\_button\_add}}(win);}
\DoxyCodeLine{   elm\_object\_text\_set(btn, \textcolor{stringliteral}{"{}Button 3"{}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Box__Group_ga66a10dd861578521af2ab7b8c2ca65c5}{elm\_box\_pack\_end}}(box, btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Smart__Object__Group_ga18a8b179f94d21b2b09e19db11741061}{evas\_object\_smart\_callback\_add}}(btn, \textcolor{stringliteral}{"{}clicked"{}}, \_btn\_clicked\_cb, NULL);}
\DoxyCodeLine{}
\DoxyCodeLine{   btn = \mbox{\hyperlink{group__Elm__Button_ga0021b9d86d822367035843ed18bc6ea9}{elm\_button\_add}}(win);}
\DoxyCodeLine{   elm\_object\_text\_set(btn, \textcolor{stringliteral}{"{}Button 4"{}});}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__Box__Group_ga66a10dd861578521af2ab7b8c2ca65c5}{elm\_box\_pack\_end}}(box, btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(btn);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Smart__Object__Group_ga18a8b179f94d21b2b09e19db11741061}{evas\_object\_smart\_callback\_add}}(btn, \textcolor{stringliteral}{"{}clicked"{}}, \_btn\_clicked\_cb, NULL);}
\DoxyCodeLine{}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group_ga21e8604c0e4a93e469bff4bd069e82b5}{evas\_object\_resize}}(win, 300, 320);}
\DoxyCodeLine{   \mbox{\hyperlink{group__Evas__Object__Group__Basic_ga9cbc13661584e49fb9d9cdab514a1eeb}{evas\_object\_show}}(win);}
\DoxyCodeLine{}
\DoxyCodeLine{   \mbox{\hyperlink{group__Elm__General_gadf2553f83e6b0b9c7fbe054019a664a1}{elm\_run}}();}
\DoxyCodeLine{}
\DoxyCodeLine{   \textcolor{keywordflow}{return} 0;}
\DoxyCodeLine{\}}
\DoxyCodeLine{\mbox{\hyperlink{group__Elm__General_ga50fe52d513f7e89c9c770afd76de7ad9}{ELM\_MAIN}}()}

\end{DoxyCodeInclude}
 That\textquotesingle{}s pretty much it. The full code is \mbox{\hyperlink{theme_example_02_8c-example}{here}} and the definition of the theme is the same as before, and can be found in \mbox{\hyperlink{theme_example_8edc-example}{here}}. 