This is really a question for the theme in question, this forum is really meant to be for help with BP and it’s default theme.
But I see that you have already asked this exact same question :
https://wordpress.org/support/topic/frisco-theme-sidebar-of-width-300
So I suggest that you wait for a reply from that post firstly, cross posting the same question on multiple forums is considered a bad practise, if the theme author cannot help after a reasonable period then ask the question here or on other tech forums that might be able to help.