Wrong LESS conversion

- Gitea version (or commit ref): master (31d54880598beb7fdc9832147521063292ae9c0c)

public/css/index.css contains style like:
```
background:#21BA4518 18
```
or
```
border-left:1px solid #21BA4580 80
```

They are not valid CSS syntax.
Maybe LESS.js (3.6.0) generates wrong code for rgba value.