Remove indent after paragraph

allParagraphStyles gives Array of objects not a collection, so item(), firstItem(), ... etc doesn't works. Use paragraphStyleGroups.item("YourGroupName") before paragraphStyles:

line3:

app.findTextPreferences.appliedParagraphStyle

= app.activeDocument.paragraphStyleGroups.item("YourGroupName").paragraphStyles.item("Mellanrubrik");

That's a reasonable starting point. First off, you cannot simply use the name of your style; you need to 'look it up' using "app.activeDocument.paragraphStyles.item("Mellanrubrik")". Second, if you use the Find function to locate your heads, the 'selection' will not 'hold' the next paragraph. There are several ways to increment the, uh, 'cursor' (not really) to the next paragraph, but the easiest way is to use 'nextItem' on the Found Item's paragraph object.

With those adjustments, your little scriptlet works once only, and the next step would be to make it work on an entire document:

1. Find all headlines.

2. For each of the found headlines,

   a. Check if the next paragraph is your body text (I'd advise this check)

   b. If so, set its indent to '0'.

(Fill in the name of your body text paragraph style below.)

// 1. Find a list of all headers
app.findTextPreferences = null;
app.findTextPreferences.appliedParagraphStyle = app.activeDocument.paragraphStyles.item("Mellanrubrik");
list = app.activeDocument.findText(0);
// 2. For each found item ...
for (i=0; i<list.length; i++)
{
// a. Check the *next* paragraph for your body text
if (list.paragraphs.nextItem(list.paragraphs[0]).appliedParagraphStyle == app.activeDocument.paragraphStyles.item("Body Text"))
// b. If so, remove indent
  list.paragraphs.nextItem(list.paragraphs[0]).firstLineIndent = 0;
}

A disadvantage, however, would be that moving the headers around will also have left other paragraphs without an indent that should have one!

You could consider to duplicate your body text style, set the indent to 0, and name it something like "Body Text After Head" -- that way you don't get a nasty "override +" -- and apply this instead of the override. When you notice someone else moved stuff around, delete this new style (!!) and replace it with your plain body text. Then re-create the style and run the script again.