I'm trying to make a sidebar/panel to filter products and overall I use minimalist/flat design. Here is the layout, collapsed and expanded.
My problem is, as you can see, the accordion button (filter button) is somewhat invisible, especially when it is expanded. I want users to realize that it can be collapsed again by clicking it again. Do you have any idea about its design to make it bold but still simple/minimalist? I don't think I can use regular rounded button design because it's not quite a button (?).
Thank you, any idea appreciated! :)