The places you can set look of your admin area:

Theme manager

When you select the Admin theme option, you can select the admin layout style. Flex panel is recommended (or info panel) 

Personalize

On the right side, in user options, you can personalize your dashboard look:

• dashboard layout (admin UI has it is own layout, so default is enough)
• icons on the dashboard. The core limitation is 20 icons. yes, it should be 24 to look good at all devices.
• menus on the left side of the dashboard. It is not menus for the menu manager, only for displaying menus on the left dashboard pane. This way you can create a menu in the plugin and put there everything you need to be displayed on the dashboard. 

Preferences / Admin display

• Separate plugins into their own menu. - normally you can set in plugin.xml or plugin manager where plugin config is displayed. With this option, all plugins are under separate admin option Plugins
• Display navigation-bar labels - the main menu displays Labels, not only icons (like Preferences, Users, Content, etc...) Recommended is ON. 
• Display field help tips - it displays a little help icon on the left side of form fields. With hover, you can see help for this field.
• Collapse navigation side-bar by default - the left sidebar is changed to icons by default (to open it, you need to click on little help icon on the bottom of page). Never used.

 No idea what other settings mean:

• Display plugins info: Will display info on all admin pages for each plugin supporting this type of feature
• Unique 'Plugins info' menu: If disabled, each plugin will display its own info in an individual menu. If enabled all info will be displayed in one menu.

There was a special plugin for more customization, but it is not supported for 2.3.1 (some of the functionality is already in core, some are related to white labeling and with new admin skins styling is outdated). Not recommended to use for now (or without styling).