[Theme: TwentyFifteen] How to expand main frame and adjust menu

Hello,

First off, I tried everything from https://www.ads-software.com/support/topic/reducing-width-of-sidebar-of-twenty-fiteen-theme?replies=21 and nothing. Nada.

On https://www.clairechauvel.fr/ and https://samanthahahn.com/ they have a nice large main page width.. nice galleries. Nice left-hand menu with smaller text..also less spacing on their website text. How to I get both their frame sizes, menu text/etc. sizes and their lightbox ?

My website is https://danaburns.net .

Thanks in advance.