Working with engineers of b2b saas companies every day, for more than a year and having analysed all the best SaaS companies who have 10+ internal designers, I found a couple of principles that anyone can apply to make their website look decent:
* Consistency - One practical example: If you use a 4px border-radius, use a 4px radius everywhere. It may sound small, but having a consistent experience across your application makes the product feel so much more polished to your users. Don't use multiple fonts, different navigation menus etc. Keep it consistent.
* Reduction - If anything, design isn't about adding more things to make it 'look nice'. Try to remove as many things as you can. If something doesn't serve a specific function, then remove it. Every pixel should make sense. The more you put in front of your users, the more brain power it'll require to process.
* Divide - This is mostly UX, but one thing I see so many get wrong. A lot of SaaS apps overwhelm their users. They present them with all the features upfront. Whether it's a long onboarding form, or a dashboard with 50 actions one could take. By splitting up things in different ways, you can guide the user through the experience. Your signup process for example (that might be a big block in conversion) might be made so much better if you ask for certain types of information later on in the process.