hseperation is the distance between the text and icon of a tab, see this issue. If you want to create gaps between the tabs you've to edit the theme:
- Select the TabContainer and search the property inspector for "Theme"
- Choose "Create New Theme" and a new window at the bottom will open
- In that window, choose Edit Theme > Add Class Items > TabContainer
- Back in the inspector, unfold Tab Container > Styles
- Choose New StyleBoxFlat for "Tab Bg"
- Now change Border Width > Left / Right, e.g. to 100
- Then set Border > Color to fully transparent (i.e. the A(lpha)-value to 0)
- For more inner space, change Expand Margin > Left / Right
- Repeat steps 5-8 with "Tab Disabled" and "Tab Fg"