Thanks a lot for your reply. I would love to open an issue and contribute.

I have actually downloaded from the project Github repo at my local but cannot run it. The contribution doc says I need yarn which I don't have. I however, have npm. How can I run the dev server and run the project using npm? Please help.

Which repo do you refer to? Bootstrap or BootstrapVue?

Whilenpm isn't recommended to use, it's still possible.
After cloning the repo, you should be able to runnpm install in the cloned folder and thennpm run docs-dev to launch the docs locally.
You can then uselocalhost:3000/play to play around with changes locally.

But i do recommend you to install yarn, there isn't much difference syntax wise to NPM, and some scripts are based around them.
It's also a good thing to learn as it's widely used.

These variables are defined in Bootstraps_variables.scss file, which contains all the variables (with default values) that they use in their project. You can see this filehere.
Be aware thatBootstrapVue only supportsBootstrap v4 right now, and that the mainBootstrap branch is for v5.
So if you go looking, remember to go on the v4-dev branch, or the latest v4 tag when viewing theBootstrap repo.

BootstrapVue has some custom SCSS variables, which are contained in it's own_variables files. You can view this filehere.

Just be aware that the specific variable$nav-link-disabled-color, that I mentioned is from theBootstrap repo and notBootstrapVue, that you're currently looking at.
So if you want the default value changed, you should not submit a PR toBootstrapVue, but insteadBootstrap, as we getmost of the CSS from them. (hence the name).

If I have understood correctly, if I want this colour to change, I need to work on Bootstrap v4 repo and submit a PR there. If it is accepted, BootstrapVue will automatically be updated as well since it is fetching these variables directly from Bootstrap?

Becausehttps://github.com/twbs/bootstrap/blob/v4.5.2/scss/_variables.scss has$nav-link-disabled-color which is using$gray-600 which is actually$gray-600: #6c757d !default; in the same file.

If I have understood correctly, if I want this colour to change, I need to work on Bootstrap v4 repo and submit a PR there. If it is accepted, BootstrapVue will automatically be updated as well since it is fetching these variables directly from Bootstrap?

Basically, yes.

However, SCSS allows you to change it freely in your own project without submitting any PRs to Bootstrap.
You can change all the variables you want in your own project. Bootstrap simply supplies some default values that can be changed by the developer in their own projects.

You can read more info on the docs on how to use SCSS in your own project:
Setup:https://bootstrap-vue.org/docs#using-module-bundlers
Theming:https://bootstrap-vue.org/docs/reference/theming

Additional info on theming:https://getbootstrap.com/docs/4.5/getting-started/theming/

Thank you for you help! I have created a feature request at Bootstraphere