style sheet for which you want to edit space:
Go to space, then select Space Tools > Interface appearance in the sidebar on the left (spaces tool > Look and Feel).
Select the style sheet (Stylesheet) and select edit.
Paste your custom CSS style sheet file in.
Save your settings. The new CSS style sheet file will be applied to all content pages in the space.
If you are using a document theme:
Select Browse > Space management from the top (Browse > Spaces Admin) Note: Space Admin will only appear if you have space administrators and their permissions or if you are a member of the Confluence Admins group.
Click the style sheet (Stylesheet)defined in the Appearance and interface (look and feel) under the left panel.
Select Edit.
Paste your custom CSS style sheet file in.
Save your settings. The new CSS style sheet file will be applied to all content pages in the space.
To edit your global CSS style sheet:
In the upper-right corner of the screen, click the console button , and then select the general Configuration link.
Select the style sheet (Stylesheet).
Select Edit.
Post your custom CSS style sheet into the text box.
Select Save.
Navigating the global style sheet
Editing interface for global style sheets
Note:
The new CSS style sheet will work in all of the global space, providing a CCTV table for spaces that do not have a custom style sheet. The style sheet for this place will overwrite all global style sheets.
You can add your own CSS style sheet by customizing HTML (custom HTML) , which defines the HEAD or BODY of the page. You should use this option when using a global stylesheet that does not achieve your purpose.
Refer to related guidelines
Please refer to some examples in Basic styling Tutorial to start customizing your CSS.
Https://www.cwiki.us/display/CONFLUENCEWIKI/Styling+Confluence+with+CSS
Confluence 6 start editing CSS