From The Document Foundation Wiki
- create a new style, for example Header1bis, based on Header 1;
- set the style to the "Introduction", "Conclusion" and "Résumé" header;
- right click the table of contents and select Edit index;
- under the Type tab, check Additional styles and click Assign styles...;
- in the Additional style window that opens, select the new style Header1bis;
- set the required level for the table of contents using the radio buttons: choose a level that is not used, for example 10;
- press OK;