Contrasted triangles on interface and transparent materials in viewport and material view System Information
Operating system: Linux-5.8.0-25-generic-x86_64-with-debian-bullseye-sid 64 Bits
Graphics card: AMD TURKS (DRM 2.50.0 / 5.8.0-25-generic, LLVM 11.0.0) X.Org 3.3 (Core Profile) Mesa 20.2.1
Blender Version
Broken: version: 2.90.1, branch: master, commit date: 2020-09-23 06:43, hash: rB3e85bb34d0d7
Worked: (newest version of Blender that worked as expected)
Short description of error
I get contrasted triangles throughout the interface, everywhere. Ill attach screenshot to understand the issue better. Some materials are also displayed transparent so matter what settings I tweak, and sometimes the model appears to have many black triangles inside it where there is no mesh, inside the empty spaces. I am sure this a graphical issue, but I have tried to install additional drivers and drivers from official sites and ppa's but nothing has worked.
TL;DR: Your GPU does not fulfill the minimum requirements for running Blender 2.8x or later.

&quot;Turks&quot; is part of the codename of multiple GPUs that have the TeraScale 2 micro-architecture:

Radeon HD 6570 (Turks Pro)
Radeon HD 6570 (Turks Pro)
Radeon HD 6670 (Turks XT)
Radeon HD 7510 (Turks LE)
Radeon HD 7570 (Turks Pro-L)
Radeon HD 7670 (Turks XT)
FirePro V3900 (Turks GL)
FirePro V4900 (Turks XT GL)
FirePro M2000 (Turks GL)

In order to run Blender properly, your hardware has to fulfill the minimum requirements. For AMD graphics cards the micro-architecture has to be at least GCN first generation or later. This is to ensure normal operation, GPU rendering has higher requirements. At the time of writing the minimum requirements for graphics card for Blender 2.8x and 2.9x are:

Supported Graphics Cards

NVIDIA: GeForce 400 and newer, Quadro Tesla GPU architecture and newer, including RTX-based cards, with NVIDIA drivers (list of all GeForce and Quadro GPUs)
AMD: GCN 1st gen and newer (list of all AMD GPUs)
Intel: Haswell and newer (list of all Intel GPUs)
macOS: version 10.13 or newer with supported hardware


Unfortunately, this means that your GPU is below the minimum requirements and not officially supported. You may still be able to run Blender, but graphics glitches or even crashes, due to issues with the graphics driver, may occur. Sometimes the graphic glitches can be resolved by installing a more recent graphics driver, if one is available.
Blender 2.79b is the last release that officially supported your GPU. Previous versions of Blender can be download from this page.