Hi,
If you could give me more details of the page you where at when you took the screen shot, as I am not familiar with that page at all, that would be a great help?
Many of the Report pages and Admin pages in Moodle may well be in need of more scrolling, but unless we get a bug reported in Moodle Tracker to start tracking these kind of bugs then we can't do anything about them.
Adding some custom CSS is fine providing you add it to the theme that you are using. Not every theme has a base.css file most themes carry a file called core.css.
Then again, some themes like Afterburner have a custom css settings page built into the theme, this allows you to add css to fix a problem or change the look of the theme.
So with regards this issue, first we need more information, Moodle version and theme name, and lastly the location of the page that has the problem, or list of pages that have the overflow problems.
Oh...and here is a quick fix using the browser Zoom out using feature. You can either use the browser View/Zoom/'Zoom out' or use your keyboard, Ctrl + - (Control key plus the minus key).
Hope this helps?
Mary