I want to add a little rounding on my buttons using the command border-radius: 10px;. What is the best place to put it for all buttons in the files of my used template Bike?

Please analyze the css.
www.joomlaplates.com/template-customizing
And then put the border into your css/custom.css

ok thanks, if I leave the option "Compile LESS in the browswer on every page load." unchecked it works but then I get an error message when going to the login menu.

Below the error message. Probable because I changed something in another file.
Error: Class 'Warp\Asset\Filter\Exception' not found: Class 'Warp\Asset\Filter\Exception' not found

If I leave that function checked, the change in custom css does not work.

1. LESS Developer Mode

Check the first option and the browser automatically compiles your LESS files via JavaScript every time you load a page. This provides an appropriate testing environment. It's not recommended to enable developer mode on the live site as the constant process of compiling is highly inefficient.

www.joomlaplates.com/support/documentation/warp-7-settings

Do you have other compression tools?
Otherwise update the Warp7
www.joomlaplates.com/support/documentati...e-the-warp-framework