I Hacked Gmail with an Inbox-Inspired Theme :D

usercss-hacked-gmail.css

/*--- #3. Hack the Tab Labels ---*/

/* Hide Ugly Tab Labels */
.aAy.aIf-aLe .aKx .aKz,
.aAy.aJi-aLe .aKx .aKz,
.aAy.aH2-aLe .aKx .aKz,
.aAy.aHE-aLe .aKx .aKz{
  display: none;
}

/* New Tab Label Font */
.aAy.aIf-aLe .aKx:after,
.aAy.aJi-aLe .aKx:after,
.aAy.aH2-aLe .aKx:after,
.aAy.aHE-aLe .aKx:after{
  display:block;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
  -webkit-font-smoothing: antialiased;
  font-family: 'Google Sans',Roboto,RobotoDraft,Helvetica,Arial,sans-serif;
  font-size: .875rem;
  color: #5f6368;
  font-weight: 500;
  letter-spacing: 0;
  line-height: 16px;
  order: 0;
}

/* Tab: Active Font Color */
.aAy.aIf-aLe.J-KU-KO .aKx:after,
.aAy.aJi-aLe.J-KU-KO .aKx:after,
.aAy.aH2-aLe.J-KU-KO .aKx:after,
.aAy.aHE-aLe.J-KU-KO .aKx:after{
  color: #3eb1ad;
}

/* Tab: Active Underline Color */
.aAy.aIf-aLe.J-KU-KO:before,
.aAy.aJi-aLe.J-KU-KO:before,
.aAy.aH2-aLe.J-KU-KO:before,
.aAy.aHE-aLe.J-KU-KO:before{
  background-color: #3eb1ad;
}

/* Tab: Hides "New" Count */
.aKx>.aDG{
  display: none;
}

.aJi-aLe .aDG, 
.aAy.J-KU-JW.aAy.aJi-aLe .aKo, 
.aAy.J-KU-KO.aAy.aJi-aLe .aKo{
   display: none;
  
}

/* Tab: Important */
.aAy.aIf-aLe .aKx:after{
  content: "Important";  
}

/* Tab: Important Icon */
.aAy.aIf-aLe .aKp.aIf-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553030473/media/email_t4wvmi.svg');
  background-size:18px;
}

/* Tab: Important Icon Active */
.aAy.aIf-aLe.J-KU-KO  .aKp.aIf-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553032741/media/email-teal_ip8ci6.svg');
  background-size:18px;
}

/* Tab: Fun & Events */
.aAy.aJi-aLe .aKx:after{
  content: "Fun & Events";
}

/* Tab: Fun & Events Icon */
.aAy.aJi-aLe .aKp.aJi-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553031289/media/music_maycua.svg');
  background-size:18px;
}

/* Tab: Fun & Events Icon Active */
.aAy.aJi-aLe.J-KU-KO .aKp.aJi-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553032741/media/music-teal_lho3c2.svg');
  background-size:18px;
}

/* Tab: Work & Receipts */
.aAy.aH2-aLe .aKx:after{
  content: "Work & Receipts";
}

/* Tab: Work & Receipts Icon */
.aAy.aH2-aLe .aKp.aH2-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553033425/media/suitcase_yrfiac.svg');
  background-size:22px;
}

/* Tab: Work & Receipts Icon Active */
.aAy.aH2-aLe.J-KU-KO .aKp.aH2-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553033425/media/suitcase-teal_lf8pmp.svg');
  background-size:22px;
}

/* Tab: Github */
.aAy.aHE-aLe .aKx:after{
  content: "Github";
}

/* Tab: Github Icon */
.aAy.aHE-aLe .aKp.aHE-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553033424/media/github_efugnj.svg');
  background-size:20px;
}

/* Tab: Github Icon Active */
.aAy.aHE-aLe.J-KU-KO .aKp.aHE-aLf{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553033424/media/github-teal_mk16yp.svg');
  background-size:20px;
}

/*--- #4. Get rid of Hangouts ---*/



/*--- #5. Get rid of Ads ---*/
.BltHke.nH.oy8Mbf.aE3 .aKB{
  display: none;
}

/*--- #5.1 Nov 5, 2019 Google updated & ads are showing again. ---*/
.aDP .ae4.aDM{
  /* border: 1px solid pink; */
}

.aDP .ae4.aDM + .ae4.aDM{
  /* border: 1px solid orange; */
}

.aDP .ae4.aDM + .ae4.aDM + .ae4.aDM{
  /* border: 1px solid yellow; */
}

.aDP .ae4.aDM + .ae4.aDM + .ae4.aDM + .ae4.aDM{
  /* border: 1px solid green; */
}

/*--- June 20, 2020 Google updated & it causes this whole block to be hidden if targeted this way. 
.aDP .ae4.aDM + .ae4.aDM .Cp{
  display: none;
}
THIS NO LONGER WORKS ---*/

/*--- #5.2 June 20, 2020 ---*/
.aDP .ae4.aDM + .ae4.aDM .Cp + .Cp{
  display: none;
}

.aDP .ae4.aDM + .ae4.aDM .Cp + div + .Cp{
  /* border: none; */
  display: block;
}

.aDP .ae4.aDM + .ae4.aDM + .ae4.aDM .Cp{
  /* border: none; */
}

.aDP .ae4.aDM + .ae4.aDM + .ae4.aDM + .ae4.aDM .Cp{
  /* border: none; */
}

/*--- #6. Even MORE line-height for Emails ---*/
.Cp tr{
    padding-top: 18px;
    padding-bottom: 18px;
}

/*--- #7. Minimal Sidebar ---*/

/* Change Inbox icon to Envelope Icon */
.aHS-bnt .qj {
    background-image: url('https://res.cloudinary.com/airship/image/upload/v1553030473/media/email_t4wvmi.svg') !important;
    background-size: 18px !important;
}

.nZ>.aHS-bnt .qj, 
.aHS-bnt .qj::before{
  background-image: url('https://res.cloudinary.com/airship/image/upload/v1553030473/media/email_t4wvmi.svg')  !important;
  background-size: 18px !important;
}

/* Set Grey Sidebar Highlight */
.byl .TO.nZ>.aHS-bnt .n0, 
.nZ>.aHS-bnt .bsU{
  color: #333333 !important;
}

.wT>.n3 .byl:first-child .aim:first-child .nZ {
  background-color: #e8eaed !important;
}

/* Hide Sidebar Label Icons */
.zw .TK .qj.qr{
  display: none;
}

/* Add space between sidebar labels */
.aim, .ain {
    padding-top: 2px;
    padding-bottom: 2px;
    padding-left: 0;
}

/* Simplify "More" */
.n6 span.ait {
    display: none;
}

/* Adjust Compose Button Margin */
.aeN .z0>.L3{
  margin-left: 8px;
}

/*--- #8. Get rid of Notification Dots and Unread Counts ---*/
.zw .TK .aio.aip .bsU{
  display: none;
}

/*--- #11. Bring back the delightful cleared inbox graphic ---*/
.DVI7hd .aRu{
  background-image: url('https://res.cloudinary.com/airship/image/upload/f_auto/v1553218540/media/airship-unicorny_gpgyga.png');
  background-size: 240px auto;
  background-repeat: no-repeat;
  background-position: center center;
  height: 200px;
  margin-top: 40px;
}

.DVI7hd .aRu .aRv{
  display: none;
}

.DVI7hd .aRu .aRv + p{
  display: none;
}

.DVI7hd .aRu .aRv + p + p{
  display: none;
}

/*--- #12. Use "Product Sans" in the sidebar ---*/
/* Set Sidebar Label Font */
.TO .nU>.n0, .TO.NQ .nU>.n0, 
.TO.nZ .nU>.n0, .ah9>.CJ, 
.n3>.CL>.CK{
  font-family: 'Google Sans',Roboto,RobotoDraft,Helvetica,Arial,sans-serif;
  font-weight: 500;
  font-size: .85rem;
  color: #333;
}

/* Hide Gmail Logo */
.gb_ja:not(.gb_ka) .gb_qc .gb_Rb,
.a.gb_Vd.gb_Tb.gb_ce{
  display: none !important;
}

/* Hides Gmail Footer */
.l2.pfiaof {
  display: none;
}

/*--- #13. Unicorn empty state for split screen view. ---*/
.apb, .apO{
  display: none;
}

.apa{
  background-image: url('https://res.cloudinary.com/airship/image/upload/f_auto/v1553218540/media/airship-unicorny_gpgyga.png');
  background-size: 275px auto;
  background-repeat: no-repeat;
  background-position: center 80px;
  height: 100%;
}

Here is the link for the chrome extension - https://chrome.google.com/webstore/detail/unicorny-gmail/pmffbbfjmlgobdognnccbkieanlkkjjk

@g-rohit thanks for including that link!!! :D 👾 I also just updated the snippet to include additions to hide ads again. Last month, Google updated their code to make it harder to hide ads.

Also, the original blog post:
https://medium.com/@airshipcms/gmail-ux-sucks-so-i-fixed-it-for-my-own-sanity-b6f33898ebe3

@kellishouts yeah it's hard indeed to keep it up to date coz Gmail keeps changing their class names for security reasons. But, must appreciate your hard work on this.

Nice

In Firefox I use this extension: Custom Style Script

Also, here's the GitHub Repo for the Chrome Extension:
https://github.com/kellishouts/unicorny-gmail