Sometimes I feel like I am behind with CSS. I have not used LESS or SASS or any others on any production application that I work on. It's usually Bootstrap and then custom CSS to override. I have messed around and learned the basics of LESS/SASS but nothing beyond that.

Am I behind?

"Behind" feels like the wrong term, but you're definitely holding yourself back.

The good thing about SASS (or, well, SCSS, which is what you would want to use) is that it integrates directly with CSS, so you can start utilizing it in your existing CSS files without having to convert the stylesheets to SCSS. You would just need to run those files through the preprocessor.

There's a lot to learn and a lot you can do, but you can always start with the simple stuff (such as using variables to collect all color codes in one place, etc), and gradually introduce more SASS code as you learn about it and have the time to implement it.

No. I tell anyone I've run into that if you've never used SASS but you're comfortable with CSS then you'll be productive almost immediately. With a little bit of direction and reading you'll be up-to-speed within a couple of days. SASS is simply not a difficult thing to pick up.

But there's nothing holding you back from playing with it right now with Codepen or jsFiddle.

If you're working on large sites, I'd looking into SASS and modular CSS:

http://thesassway.com/modular-css

For your own projects it doesn't matter, as long as you're productive. If you're looking for employment in the frontend-sector, you should get used to SASS.

Except when it comes to writing CSS, it doesn't matter how skilled you are at CSS, one simply can not be as equally productive without using a preprocessor.

Why not start slowly by utilizing Bootstrap SASS + your custom css as SASS?

Because it is more simple to just use the prebuilt (no need to convert to css, no build tools), because browsers already cache the prebuilt? Because CDNs already have the prebuilt version. Because changing the version is about 1 character change in your own code. Because for whatever reason, some SASS stuff are not backward compatible. And most importantly none of the end users cares about it.

Probably not, but how are you handling theming?

Filled out your survey, looking forward to the results. Here is another one that was taken earlier this year for reference. http://stateofjs.com/

Not a troll attempt, just a simple question from a guy who stopped to care about frontend a while ago : isn't web tech badly designed because of successive feature bendings, and aren't all those hyped frameworks, transpilers, package managers and stuff just attempts to fix it ? The X Window System got it right back in the 80's, I don't understand what's going on here.