From The Document Foundation Wiki
How do I make the title of the table of contents look like the Header1 titles?
By default, the title of the table of contents has the style Content header. It is therefore necessary to assign a new style corresponding to the desired appearance:
- right click on the table of contents and select Edit index/table, chose the Styles tab;
- in the Levels area, select the Title level;
- in the Paragraph style area, chose the style you want to assign, for example Header1;
- click on the < button to assign it;
- click OK.