Actually it works but it's considered as anti pattern and is not recommended. Salesforce is especially warning you to do it.
There is a golden rule: don't touch what you don't have created! and to achieve what you want, you will have to break that rule.
This caveat emptor attached, you can hang yourself like this:
- create a new lightning component (you need only the markup, no helper, no controller)
- this compo will become something invisible for itself and will be used to hack the other compo
- inspect the original lightning compo you want to modify (press F12, use browser inspect tools)
- find out the css classes you want to override and figure out the relevant css selectors
- tweak the existing css in the browser tool to get a preview and understand what classes do what (this will be different for each original compo)
- add a
<style> tag into the markup of your own compo
- add selectors you want to hack to that style tag
- add some extra selectors in the front of each selector to give your selectors more prio than the original like e.g.
html body .whatEverOriginalSelector in front
- your compo is invisible but put it in the Lightning App Builder below the original compo
- your style with higher prio selectors will override the original component style
- expect issues if the original author or salesforce are changing the markup: then your hack is likely to break and you have to find new working selectors again
Be always aware this is a hack and it might break any time.
Honestly, I'm using this workaround in some use-cases. Mostly to change Saleforce default styles of out-of-the box things, e.g. some paddings are still way too large for the taste of lots of German clients.
On Record Pages worst thing I've seen happening is that the style falls back to the original. Real mess-ups I have not encountered - however they might be possible, but rare.
Be aware, that LEX is a SPA (single page app) so that an injected style will not be limited to the record page where you use it! It will get carried around and might apply on other flexipages, too. This fact can be irritating and this either can be what you want or not. I always try to be very specific in the selectors to keep a close control on what gets the styles and I only use it, if I can accept the style override on all pages and need not to limit it to the one. However even that can be accomplished using Custom Lightning Page Template. You can put styles or wrapping divs in there, too - so in combination this can be very powerful.
But still - it stays a hack.