Why style-l.css file is restore to its default file in magento2.2.2? How can I stop it?

[ParamAyurveda]

Whenever I run "php bin/magento setup:upgrade" commmand on ssh for my website which is running on magento 2.2.2. It restore style-l.css file (pub/static/frontend/Magento/luma/en_US/css/style-l.css) to it default file, means it remove all the editing which I done in this file.

What are the exact reason behind this? How can I stop it?

[Tom Robertshaw]

Have you had a look at the frontend development guide? It sounds like you might be editing the wrong file. You'll need to edit the source file in your theme http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-topics/css-overview.html

[ParamAyurveda]

Sorry Tom,

I didn't get you.

Can you please explain me, How can I edit css for front end module? Like If, I want to change color of top menu from grey to red then what should I do?

[Tom Robertshaw]

If you haven't already created your own theme, I would start there. This should be used to hold all of the changes that you want to make. 

There's some great documentation on this page for doing that http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/theme-create.html

[ParamAyurveda]

Hey,

Thanks for reply.

I have already created my own theme. 

But, where can i put my style file. Suppose I make a "custom.css" file. So where should i have to place it ?

[Tom Robertshaw · Accepted solution]

This screenshot on that docs page gives some indication to the folder structure:

Then on this documentation page: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/debug-theme.html

There is a section about locating styles which is quite descriptive. It further provides a link to http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-topics/css-themes.html

These pages all describe it far better than I could so I hope it helps.