Bootstrap 4 Sass - changing theme dynamically - css

Question

New to sass I stuck with the problem how to enable the dynamic change of a website theme - lets say a dark and a light theme - through user interaction. The template I use as a base (coreui) has a _custom.scss file in which various variables are defined
...
$navbar-bg: #fff;
...
The values of these variables would need to be set dynamically depending on the user choice of the theme and I have no clue how to get this done. Any pointer how to implement this would be highly appreciated.

Answer 1

SASS is a preprocessor which means essentially it gets compiled down into regular CSS and then shipped to the client. If you want to change themes you'll have to dynamically load different stylesheets using javascript.
Case 1
In the case that you want the user to pick between multiple prepackaged themes. This can easily be done with multiple "theme" style sheets which import the various parts of your style. First import the different colors, then import the main bodies of your sass. For example:
theme1.sass:
#import 'theme1';
#import 'base';
#import 'other';
theme2.sass:
#import 'theme2';
#import 'base';
#import 'other';
Then in javascript you could remove the old stylesheet and add the new one when the user does whatever is needed to change the theme. For example inside the onclick of a button you could put:
document.getElementById('cssTheme').setAttribute("href", "/path/to/theme");
It's probably best to take a bit of care and put the element in the head of the document, but this is a good starting point. That could be made to look a lot nicer with Jquery, but I didn't want to assume you'd have that.
Case 2
In the case that you want the user to dynamically change colors of individual element colors it might be worth looking into CSS Variables. Current support in IE/Edge is crumby but it is pretty interesting. Then as the user changes the fields you could just be changing the css variable in a <style> tag somewhere on the page and it should propagate through the document.
If you want more browser support then I think really the best way would be with OK sure's answer. This one gives you the benefit of just changing a variable and not having to reset each element style that uses that variable.

Answer 2

You have 2 options I think.
Option 1) Recompile the styles whenever a change is made by running a command serverside to generate a new CSS file for the user. This will be potentially very resource hungry and probably not recommended.
Option 2) Take the variables you want to be accessible, find where they are mentioned in the bootstrap source and either generate a file or just inline these styles after the stylesheet is included in the template.
So for your example here, depending what language you're coding in, or templating (this is a twig example) you're using, you could inline the following in the head of your template:
<style>
.navbar {
background-color: {{ user_theme.navbar-bg | default('#eeeeee') }}
}
</style>
It's tough to tell you exactly how to do this without knowing what frameworks/languages/infrastructure you're using.
If you were using Twig & PHP for example, you could send a user_theme object to the template, and have an include file which contains all the styles that need modifying with default values as above.