You have some hints want you can change with theming:
If you know HTML/PHP you can also modify the whole login page yourself. If there are parts not covered by theming, you need to apply these changes after every update/upgrade.
Versions that supported Windows or could be run on Windows are no longer supported. There are tons of bugs, security issues on top of the problems caused by Windows (which were the reason to drop Win support). If you want continue using ownCloud, you should first get a recent version and run it on a suitable OS. You can start in VirtualMachines which are great to get familiar with new OS.