Knowledge Reserve of CSS
If you have no relevant knowledge about CSS, please refer to the CSS Resources section of the page. Before you start making changes to Confluence's stylesheets, you should have some knowledge and knowledge about CSS.
Safety
Custom CSS may be injected into the page script, there is a risk of cross-site scripting (XSS). When the custom script upload function is enabled, space administrators can upload CSS style sheets with their own username and password, and then trigger the browser to run scripts when the page loads. These scripts may not even be known to the space administrators themselves. Even worse can get administrator privileges. It is for this reason that this feature is disabled by default. Confluence administrators should only enable CSS files they are familiar with, and try to avoid using CSS files of unknown origin to avoid security risks.
zoom
Every page needs to be scaled. Based on the browser each user is using, the content of the page should be able to scale correctly based on different browsers. Your CSS should be designed to take this into account. Try to do some scaling tests on Confluence's pages, this test needs to test many pages in the space, such as drafts, tags, page inheritance, etc. The stylesheet you design should work in all situations, not just the first page.
Features that cannot be disabled
Closing some links, headers, or even entire menus is very easy, you just need to set the stylesheet to 'hidden'. This may be of some help to users who are not very specific to Wikis. Simple UI design can better help users to use. Note that the fact that you hide these links does not mean that these functions do not exist in the system. Users can also re-use these functions by modifying their browser's style sheets or obtain URLs for access. Don't expect to use CSS to disable some of Confluence's features for you.
Features that should not be disabled
Users of Confluence expect related actions to be displayed in the same place at the same time. Removing or hiding buttons from Confluence pages will hinder access and use of Confluence by users who may bypass your disabling of the buttons via some specific URL.
Admin UI where custom CSS cannot be applied
The use of any custom CSS for your site will not affect the sysadmin console. The intent of this setting is that custom CSS cannot be used to prevent the functionality of the user method Confluence console.
Confluence version compatibility
When you plan to upgrade your Confluence, you need to be careful. Subsequent versions of Confluence may not be compatible with your current custom CSS - this may break your CSS, which will require you to maintain your custom CSS files after upgrading Confluence. Please ask your Confluence administrator for more information.
Test in different browsers
You should test your CSS modified Confluence site in different browsers. Internet Explorer, Chrome, Firefox, Opera and Safari (Mac OS X) are the browsers used by the vast majority of users
CSS customization is not supported
Creating custom CSS may lead to a variety of potential possibilities, and Atlassian will not provide support for issues arising from or related to custom CSS.
https://www.cwiki.us/display/CONFLUENCEWIKI/Styling+Confluence+with+CSS