Unless you're the rare breed that uses Outlook, or have switched to Inbox, you're no doubt familiar with the look of Gmail's web interface. Now, through Gmail's settings, themes and some selected add-ons, you can easily transform that look to better suit your tastes.
Settings and Layout
Click on the cog icon on the right and straight away you can choose the display density (how much space Gmail puts between interface elements) -- this is usually automatically set based on the resolution of your display, but you can set it manually here too.
Then there's the choice of inbox layouts, which can be accessed from the drop-down menu by the Inbox link on the left. You can abandon the tabbed interface to show important, unread or starred messages first, or even go back to the old priority inbox look.
If you settle for the default option, you can choose the tabs that Gmail shows on screen: Click the small plus icon next to the right-hand tab to set which tabs are displayed. If you remove a tab, then its messages drift back to the Primary folder.
Head into the Settings page from the main menu (under the cog icon) and it's possible to change the number of messages on screen, turn off conversation view, switch between buttons and icons for labels, and show or hide message snippets. The Labs page is worth a look too -- there are a number of experimental tweaks that change Gmail's appearance.
If you're ready to get a little more creative with your Gmail customisations then try the Themes link from the main menu. The more modern themes are at the top, with the older classic options underneath, and there's a My Photos button if you want to pick an image of your own.
The High Contrast theme is worth a look if you want a little more of a distinction between sections, but there's plenty of choice. If you choose a newer theme (or your own picture) you can change the text shade, vignetting and image blur using the buttons at the bottom.
Blank colours are available too and you can choose More images to see hundreds of high-quality photos hand-picked by Google as suitable for use as backdrops to your messages. If you're stuck for inspiration, there's always the Random option at the end of the list.
Any changes you make are linked to your Google account -- your chosen themes will appear wherever you log in.
Third Party Add-Ons
There are a bunch of third-party extensions and add-ons you can install if you've exhausted the customisation options inside Gmail itself. Gmelius, for example, is focused on productivity features but can make some interface tweaks too (like hiding the people widget).
Enhancements for Gmail is a Chrome extension that lets you show or hide certain interface elements on Gmail for the web, and it comes with a few useful extras as well -- the plug-in can set up desktop notifications, for example. Colour Icons for Gmail, meanwhile, does exactly what you'd expect.
Google Redesigned for Firefox is another alternative, allowing you to completely change the look of Gmail (and other Google services) using CSS code added at the browser level. Some users are reporting bugs but if you can get it to work then it's impressive.