According to the Documentation,
DefaultStyleDefinitions is a global option that specifies the default stylesheet for all new notebooks.
The stylesheet must be located in a directory that is listed in StyleSheetPath.
Note that this option affects not only on-screen display, but also determines the default styles used by Export.
By convention the custom user stylesheets should be located in the user profile
directory:
dir = FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd", "StyleSheets"}];
If the directory doesn't exists one should create it:
If[! DirectoryQ[dir], CreateDirectory[dir]];
You should put your custom stylesheet "DefaultModified.nb" in this directory and then set it as the default via
CurrentValue[$FrontEnd, DefaultStyleDefinitions] = "DefaultModified.nb";
After restarting the FrontEnd all the new Notebooks will have this stylesheet by default.
It is worth to mention that according to Kuba's comment, StyleDefinitions are always set as a path relative to directories on StyleSheetPath, for example one can specify StyleDefinitions -> FrontEnd`FileName[{"Lectures"}, "styles.nb"] for a stylesheet "styles.nb" in a directory "Lectures" located wherever on StyleSheetPath. The same should be true for DefaultStyleDefinitions.
DefaultStyleDefinitionsbut I'd rename yours toDefault.nband put it in$UserBaseDirectory/...so that notebooks have a validStyleDefinitionswhen sent to others. – Kuba Nov 29 '16 at 12:23$InstallationDirectorywhile yours would be in$UserBaseDirectory. I haven't triedDefaultStyleDefinitionsbut I supposeSetOptions[$FrontEnd...should work. – Kuba Nov 29 '16 at 12:38