From The Document Foundation Wiki
The process for previous versions, still available, is an alternative:
- create a new style, for example Header1bis, based on Header 1;
- right click the table of contents and select Edit index/table;
- on the Index tab, check Additional Styles and click the corresponding icon (…);
- in the Assign style dialog that open , select the new style Header1bis;
- set the required level that it should have in the table of contents (usually 1) using the >> and << buttons;
- Click OK twice.