You can only override properties that have hooks. This is because the Shadow DOM specifically prevents components from overriding other components' styles. The documentation says:
CSS styles defined in a parent component don’t leak into a child. In our example, a p style defined in the todoApp.css style sheet doesn’t style the p element in the c-todo-item ...
Due to LWC's Shadow DOM you cannot override those values using only the class name.
CSS styles defined in a parent component don’t leak into a child
SLDS allows to override them via styling hooks. A lot of properties are setted using var() function, which get a custom property name as first parameter and a fallback (default) value.
I.E. font-size: var(--...
.You cannot override it using that selector only. You will have to be much more specific in order to override it, for example:
.containerInstanceClass div.className > div.otehrclass > div.slds-form-element__label
However, I strongly discourage you of using the !important flag in css, as it is not considered a best practice.
Instead of using the slds ...
the vertical nav blueprints have class names which you can use and toggle based on the selected tab:
...<li class="slds-nav-vertical__item slds-is-active">...
as you can see, slds-is-active indicates what tab is active, and modifies the css in consequence, so, when you click on a slds-nav-vertical__item you can toggle it.
You will need to ...
SLDS recommends using variable density classes so when you use slds-p-around__medium, It recommends using slds-var-p-around__medium. Because of this you are getting this line. This line appears if there is any error/recommendation by any extension. If you hover over that statement you will get message for that line
Some standard tokens now have variable token alternatives so that they are responsive to display density.