I recently worked hard on adding duckduckgo in search engines option in opera mini 7 java and opera mobile 12.Earlier I had tried it in opera mini 6 and opera mobile 11 but after closing of these browsers and then restarting them opera cleverly sets the search engines to defaults (Google,Amazon,IMDb,Wikipedia etc.,I had edited IMDb but it defaulted to IMDb after restart). It worked well in opera mini 4.4 (java only) where it gives you an option to add search engine but there is no option to add search engines in opera mini 7 and opera mobile 12.You can edit and delete existing search engines but they are not remembered by opera after restart.So I tried all tricks like using about:config.I had to open DDG every time from its bookmark or add it every time by editing default search engines.Finally I got it working.Here is how it works.
For Opera Mini 7 and Opera Mobile 12 (Java,Android,iOS,Symbian) :
1) Search any term with Google or any other default search engine.
2) Hold center button of D-pad on search bar(of page,not opera) or keep pressing your finger on search bar if on touch phone.
3) Select "Add Search Engine" option.
4) Now go to search engines bar of opera.Your custom search engine will be listed there.
5) Scroll down and you'll find "Manage Search Engines" option,click it.
6) Find your custom search engine and select it.
7) Select it or click edit if on non-touch phone.
8) Put Duckduckgo search query data:
add DDG or whatever you like as Title and add "http://duckduckgo.com/?q=%s" or "https://duckduckgo.com/?q=%s" (enter text inside commas only) as Address.
"https://duckduckgo.com/?kj=p&kt=e&ke=-1&ko=1&kg=p&q=%s",e.g. kj=p means header bar color will we changed from default to purple.For those who want to know more about URL parameters visit this link